* InnerConnection -> Resistance

* Wire extends Resistance
+ Voltmeter: Meter, Amperemeter, Voltmeter

src/main/java/de/ph87/electro/CONFIG.java

@@ -7,9 +7,9 @@ import static java.lang.Math.round;
 @SuppressWarnings("unused")
 public class CONFIG {
 
-  public static final double NO_RESISTANCE = 1e-12;
+  public static final double RESISTANCE_MIN = 1e-12;
 
-  public static final double MAX_RESISTANCE = 1 / NO_RESISTANCE;
+  public static final double RESISTANCE_MAX = 1 / RESISTANCE_MIN;
 
   public static final double VOLTAGE_HIGH_MIN = 0.1;
 

src/main/java/de/ph87/electro/circuit/Circuit.java

@@ -24,10 +24,6 @@ import static de.ph87.electro.CONFIG.ALIGN;
 @Slf4j
 public class Circuit {
 
-  @Setter
-  @Getter
-  private File file = null;
-
   @NonNull
   @Getter
   private final String created;
@@ -39,6 +35,10 @@ public class Circuit {
   @NonNull
   private final List<Wire> wires = new ArrayList<>();
 
+  @Setter
+  @Getter
+  private File file = null;
+
   public Circuit() {
     this.created = ZonedDateTime.now().format(DateTimeFormatter.ISO_OFFSET_DATE_TIME);
   }

src/main/java/de/ph87/electro/circuit/CircuitPanelDropTarget.java

@@ -34,6 +34,8 @@ public class CircuitPanelDropTarget extends AbstractDropTarget {
       circuitPanel.getCircuit().addPart(new Poti(aligned));
     } else if (data.equals(Voltmeter.class.getSimpleName())) {
       circuitPanel.getCircuit().addPart(new Voltmeter(aligned));
+    } else if (data.equals(Amperemeter.class.getSimpleName())) {
+      circuitPanel.getCircuit().addPart(new Amperemeter(aligned));
     } else {
       throw new RuntimeException();
     }

src/main/java/de/ph87/electro/circuit/CircuitPanelMouseAdapter.java

@@ -2,7 +2,7 @@ package de.ph87.electro.circuit;
 
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
-import de.ph87.electro.circuit.part.parts.ConnectorSub;
+import de.ph87.electro.circuit.part.parts.Connector;
 import de.ph87.electro.circuit.wire.Wire;
 import lombok.NonNull;
 import lombok.extern.slf4j.Slf4j;
@@ -144,7 +144,6 @@ class CircuitPanelMouseAdapter extends MouseAdapter {
       }
     }
 
-
     circuitPanel.repaint();
   }
 
@@ -192,7 +191,7 @@ class CircuitPanelMouseAdapter extends MouseAdapter {
   }
 
   private void wireBendToNewConnector() {
-    final ConnectorSub newConnector = circuitPanel.getCircuit().addPart(new ConnectorSub(draggingSub));
+    final Connector newConnector = circuitPanel.getCircuit().addPart(new Connector(draggingSub));
     connectIntermediate(newConnector.getNode());
   }
 
@@ -269,7 +268,7 @@ class CircuitPanelMouseAdapter extends MouseAdapter {
   }
 
   private boolean mouseClickedAction(@NonNull final MouseEvent event) {
-    if (node != null && node.getPart() instanceof ConnectorSub) {
+    if (node != null && node.getPart() instanceof Connector) {
       switch (event.getButton()) {
         case BUTTON3:
           circuitPanel.getCircuit().removePart(node.getPart());

src/main/java/de/ph87/electro/circuit/Resistance.java

@@ -0,0 +1,55 @@
+package de.ph87.electro.circuit;
+
+import de.ph87.electro.circuit.part.node.Node;
+import lombok.Getter;
+import lombok.NonNull;
+import lombok.Setter;
+import lombok.ToString;
+
+import java.util.stream.Stream;
+
+@Getter
+@ToString
+public class Resistance {
+
+  @NonNull
+  public final Node a;
+
+  @NonNull
+  public final Node b;
+
+  @Setter
+  protected double resistance;
+
+  @Setter
+  protected double current = Double.NaN;
+
+  public Resistance(@NonNull final Node a, @NonNull final Node b, final double resistance) {
+    this.a = a;
+    this.b = b;
+    this.resistance = resistance;
+  }
+
+  public Node getOpposite(final Node node) {
+    if (a == node) {
+      return b;
+    } else if (b == node) {
+      return a;
+    }
+    throw new RuntimeException();
+  }
+
+  public double getCurrent(final Node node) {
+    return node == a ? current : -current;
+  }
+
+  public Stream<Node> filterHasNode(final Node node) {
+    if (a == node) {
+      return Stream.of(b);
+    } else if (b == node) {
+      return Stream.of(a);
+    }
+    return Stream.empty();
+  }
+
+}

src/main/java/de/ph87/electro/circuit/calculation/Calculation.java

@@ -2,7 +2,7 @@ package de.ph87.electro.circuit.calculation;
 
 import de.ph87.electro.CONFIG;
 import de.ph87.electro.circuit.Circuit;
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import de.ph87.electro.circuit.part.parts.Battery;
@@ -12,7 +12,10 @@ import lombok.NonNull;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.commons.math3.linear.*;
 
-import java.util.*;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
 
 import static java.lang.Math.max;
 
@@ -72,14 +75,14 @@ public class Calculation {
 
   private void fromSchematic() {
     wires.forEach(
-      wire -> addResistor(wire.getA(), wire.getB(), CONFIG.NO_RESISTANCE)
+      wire -> addResistor(wire.getA(), wire.getB(), CONFIG.RESISTANCE_MIN)
     );
     parts.forEach(part -> {
-      for (final InnerConnection innerConnection : part.getInnerConnections()) {
+      for (final Resistance resistance : part.getResistances()) {
-        addResistor(innerConnection.a, innerConnection.b, innerConnection.resistance);
+        addResistor(resistance.a, resistance.b, resistance.getResistance());
       }
       if (part instanceof final Battery battery) {
-        final double current = battery.getVoltage() / battery.getResistance();
+        final double current = battery.getVoltage() / battery.getInnerResistance().getResistance();
         final int indexMinus = getNodeIndex(battery.getMinus());
         final int indexPlus = getNodeIndex(battery.getPlus());
         addCurrentSource(indexMinus, indexPlus, current);
@@ -90,12 +93,15 @@ public class Calculation {
   private void toSchematic() {
     wires.forEach(wire -> wire.setCurrent(getCurrent(wire)));
     parts.forEach(part -> {
+      for (final Resistance resistance : part.getResistances()) {
+        resistance.setCurrent(getCurrent(resistance));
+      }
       part.getNodes().forEach(node -> node.setVoltage(getPotential(node)));
       part.postCalculate();
     });
   }
 
-  private double getPotential(final @NonNull Node node) {
+  private double getPotential(@NonNull final Node node) {
     final int index = getNodeIndex(node);
     if (index < 0) {
       return 0; // per definition
@@ -106,11 +112,11 @@ public class Calculation {
     return potentials.getEntry(index);
   }
 
-  private double getCurrent(final Wire wire) {
+  private double getCurrent(@NonNull final Resistance resistance) {
-    final int indexA = getNodeIndex(wire.getA());
+    final int indexA = getNodeIndex(resistance.getA());
-    final int indexB = getNodeIndex(wire.getB());
+    final int indexB = getNodeIndex(resistance.getB());
-    final double conductance = indexA >= 0 && indexB >= 0 ? matrix.getEntry(indexA, indexB) : 1 / CONFIG.NO_RESISTANCE;
+    final double conductance = indexA >= 0 && indexB >= 0 ? matrix.getEntry(indexA, indexB) : 1 / CONFIG.RESISTANCE_MIN;
-    final double potentialDifference = getPotential(wire.getB()) - getPotential(wire.getA());
+    final double potentialDifference = getPotential(resistance.getB()) - getPotential(resistance.getA());
     return conductance * potentialDifference;
   }
 

src/main/java/de/ph87/electro/circuit/demo/Demos.java

@@ -17,8 +17,8 @@ public class Demos {
     final Voltmeter voltmeter = circuit.addPart(new Voltmeter(RASTER(1, 4)));
     circuit.connect(battery.getMinus(), poti.getCommon());
     circuit.connect(battery.getPlus(), poti.getEnd());
-    circuit.connect(poti.getCommon(), voltmeter.getA());
+    circuit.connect(poti.getCommon(), voltmeter.a);
-    circuit.connect(poti.getMiddle(), voltmeter.getB());
+    circuit.connect(poti.getMiddle(), voltmeter.b);
     return circuit;
   }
 

src/main/java/de/ph87/electro/circuit/part/InnerConnection.java

@@ -1,30 +0,0 @@
-package de.ph87.electro.circuit.part;
-
-import de.ph87.electro.circuit.part.node.Node;
-
-import java.util.stream.Stream;
-
-public class InnerConnection {
-
-  public final Node a;
-
-  public final Node b;
-
-  public final double resistance;
-
-  public InnerConnection(final Node a, final Node b, final double resistance) {
-    this.a = a;
-    this.b = b;
-    this.resistance = resistance;
-  }
-
-  public Stream<Node> filter(final Node node) {
-    if (a == node) {
-      return Stream.of(b);
-    } else if (b == node) {
-      return Stream.of(a);
-    }
-    return Stream.empty();
-  }
-
-}

src/main/java/de/ph87/electro/circuit/part/Part.java

@@ -1,5 +1,6 @@
 package de.ph87.electro.circuit.part;
 
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.node.Node;
 import de.ph87.electro.circuit.part.node.NodeDto;
 import de.ph87.electro.circuit.part.parts.*;
@@ -63,12 +64,15 @@ public abstract class Part {
     return switch (abstractDto) {
       case final BatteryDto dto -> new Battery(dto);
       case final LightDto dto -> new Light(dto);
-      case final ConnectorSubDto dto -> new ConnectorSub(dto);
+      case final ConnectorDto dto -> new Connector(dto);
       case final Switch1x1Dto dto -> new Switch1x1(dto);
       case final Switch1x2Dto dto -> new Switch1x2(dto);
       case final SwitchCrossDto dto -> new SwitchCross(dto);
       case final PotiDto dto -> new Poti(dto);
-      case final VoltmeterDto dto -> new Voltmeter(dto);
+      case final MeterDto dto -> switch (dto.getType()) {
+        case VOLT -> new Voltmeter(dto);
+        case AMPERE -> new Amperemeter(dto);
+      };
       case null, default -> throw new RuntimeException();
     };
   }
@@ -141,7 +145,7 @@ public abstract class Part {
   }
 
   @NonNull
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     return Collections.emptyList();
   }
 

src/main/java/de/ph87/electro/circuit/part/PartDto.java

@@ -33,12 +33,13 @@ public abstract class PartDto {
     return switch (abstractPart) {
       case final Battery part -> new BatteryDto(part);
       case final Light part -> new LightDto(part);
-      case final ConnectorSub part -> new ConnectorSubDto(part);
+      case final Connector part -> new ConnectorDto(part);
       case final Switch1x1 part -> new Switch1x1Dto(part);
       case final Switch1x2 part -> new Switch1x2Dto(part);
       case final SwitchCross part -> new SwitchCrossDto(part);
       case final Poti part -> new PotiDto(part);
-      case final Voltmeter part -> new VoltmeterDto(part);
+      case final Voltmeter part -> new MeterDto(part);
+      case final Amperemeter part -> new MeterDto(part);
       case null, default -> throw new RuntimeException();
     };
   }

src/main/java/de/ph87/electro/circuit/part/node/Node.java

@@ -38,7 +38,6 @@ public class Node {
   @NonNull
   protected Point position;
 
-  @NonNull
   @ToString.Include
   private double voltage = Double.NaN;
 
@@ -50,7 +49,7 @@ public class Node {
     this.inside = inside;
     this.uuid = UUID.randomUUID().toString();
     this.name = name;
-    this.position = part.transform(inside);
+    this.position = calculateAbsolutePosition();
   }
 
   public Node(@NonNull final Part part, @NonNull final NodeDto dto, @NonNull final Point inside) {
@@ -58,14 +57,14 @@ public class Node {
     this.name = dto.getName();
     this.part = part;
     this.inside = inside;
-    this.position = part.transform(inside);
+    this.position = calculateAbsolutePosition();
   }
 
   @NonNull
   @ToString.Include
   @SuppressWarnings("unused") // lombok toString
   public List<String> destinations() {
-    return wires.stream().map(wire -> wire.getOpposite(this)).map(Node::getUuid).toList();
+    return wires.stream().map(connection -> connection.getOpposite(this)).map(Node::getUuid).toList();
   }
 
   public void setVoltage(final double voltage) {
@@ -74,7 +73,12 @@ public class Node {
   }
 
   public void positionChanged() {
-    position = part.transform(inside);
+    position = calculateAbsolutePosition();
+  }
+
+  @NonNull
+  private Point calculateAbsolutePosition() {
+    return part.transform(inside);
   }
 
   public void draw(@NonNull final Graphics2D g) {
@@ -96,7 +100,7 @@ public class Node {
     }
     connected.add(this);
     wires.forEach(wire -> wire.getOpposite(this).collectConnectedNodes(connected));
-    part.getInnerConnections().stream().flatMap(innerConnection -> innerConnection.filter(this)).forEach(opposite -> opposite.collectConnectedNodes(connected));
+    part.getResistances().stream().flatMap(resistance -> resistance.filterHasNode(this)).forEach(opposite -> opposite.collectConnectedNodes(connected));
   }
 
 }

src/main/java/de/ph87/electro/circuit/part/parts/Amperemeter.java

@@ -0,0 +1,25 @@
+package de.ph87.electro.circuit.part.parts;
+
+import lombok.Getter;
+import lombok.NonNull;
+
+import java.awt.*;
+
+@Getter
+public class Amperemeter extends Meter {
+
+  public Amperemeter(@NonNull final Point position) {
+    super(MeterType.AMPERE, position);
+  }
+
+  public Amperemeter(@NonNull final MeterDto dto) {
+    super(dto);
+  }
+
+  @Override
+  public double getValue() {
+    final double current = innerResistance.getCurrent();
+    return Double.isNaN(current) ? 0.0 : current;
+  }
+
+}

src/main/java/de/ph87/electro/circuit/part/parts/Battery.java

@@ -1,9 +1,10 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.Setter;
 import lombok.ToString;
 
@@ -19,6 +20,10 @@ import static java.lang.Math.round;
 @ToString(callSuper = true, onlyExplicitlyIncluded = true)
 public class Battery extends Part {
 
+  public static final double DEFAULT_INNER_RESISTANCE = 0.05;
+
+  public static final int DEFAULT_VOLTAGE = 3;
+
   private static final int MINUS_W = (int) round(0.1 * RASTER);
 
   private static final int MINUS_H = (int) round(0.3 * RASTER);
@@ -29,34 +34,38 @@ public class Battery extends Part {
 
   private static final int PLUS_H = (int) round(0.6 * RASTER);
 
+  @NonNull
   private final Point size = new Point(RASTER, RASTER);
 
+  @NonNull
   private final Node minus;
 
+  @NonNull
   private final Node plus;
 
-  @Setter
+  private final Resistance innerResistance;
-  @ToString.Include
-  private double voltage = 3;
 
   @Setter
   @ToString.Include
-  private double resistance = 0.05;
+  private double voltage = DEFAULT_VOLTAGE;
 
-  public Battery(final Point position) {
+  public Battery(@NonNull final Point position) {
     super("Batterie", position);
     minus = addNode("MINUS", P10, P50);
     plus = addNode("PLUS", P90, P50);
+    innerResistance = new Resistance(minus, plus, DEFAULT_INNER_RESISTANCE);
   }
 
-  public Battery(final BatteryDto dto) {
+  public Battery(@NonNull final BatteryDto dto) {
     super(dto);
     minus = addNode(dto.getMinus(), P10, P50);
     plus = addNode(dto.getPlus(), P90, P50);
     voltage = dto.getVoltage();
+    innerResistance = new Resistance(minus, plus, dto.getResistance());
   }
 
-  public static Stream<Battery> filterCast(final Part part) {
+  @NonNull
+  public static Stream<Battery> filterCast(@NonNull final Part part) {
     if (part instanceof final Battery battery) {
       return Stream.of(battery);
     }
@@ -64,12 +73,7 @@ public class Battery extends Part {
   }
 
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  protected void _render(@NonNull final Graphics2D g) {
-    return List.of(new InnerConnection(minus, plus, resistance));
-  }
-
-  @Override
-  protected void _render(final Graphics2D g) {
     drawLine(g, P10, P50, P50 - GAP / 2 - MINUS_W / 2, P50, Color.BLACK, SYMBOL_STROKE);
     drawLine(g, P50 + GAP / 2 + PLUS_W / 2, P50, P90, P50, Color.BLACK, SYMBOL_STROKE);
     fillRect(g, P50 - MINUS_W - GAP / 2, P50 - MINUS_H / 2, MINUS_W, MINUS_H, Color.BLACK);
@@ -77,8 +81,14 @@ public class Battery extends Part {
   }
 
   @Override
-  protected void _labels(final Graphics2D g) {
+  protected void _labels(@NonNull final Graphics2D g) {
     drawText(g, LABEL_FONT, "%.1fV".formatted(voltage), P50, P10, Color.BLACK, orientation);
   }
 
+  @NonNull
+  @Override
+  public List<Resistance> getResistances() {
+    return List.of(innerResistance);
+  }
+
 }

src/main/java/de/ph87/electro/circuit/part/parts/BatteryDto.java

@@ -17,11 +17,14 @@ public class BatteryDto extends PartDto {
 
   private double voltage;
 
+  private double resistance;
+
   public BatteryDto(final Battery battery) {
     super(battery);
     this.minus = new NodeDto(battery.getMinus());
     this.plus = new NodeDto(battery.getPlus());
     this.voltage = battery.getVoltage();
+    this.resistance = battery.getInnerResistance().getResistance();
   }
 
 }

src/main/java/de/ph87/electro/circuit/part/parts/Connector.java

@@ -3,6 +3,7 @@ package de.ph87.electro.circuit.part.parts;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.ToString;
 
 import java.awt.*;
@@ -11,18 +12,20 @@ import static de.ph87.electro.CONFIG.SUB_RASTER;
 
 @Getter
 @ToString(callSuper = true)
-public class ConnectorSub extends Part {
+public class Connector extends Part {
 
+  @NonNull
   private final Node node;
 
+  @NonNull
   private final Point size = new Point(SUB_RASTER, SUB_RASTER);
 
-  public ConnectorSub(final Point position) {
+  public Connector(@NonNull final Point position) {
     super("", position);
     node = addNode("", SUB_RASTER / 2, SUB_RASTER / 2);
   }
 
-  public ConnectorSub(final ConnectorSubDto dto) {
+  public Connector(@NonNull final ConnectorDto dto) {
     super(dto);
     node = addNode(dto.getNode(), SUB_RASTER / 2, SUB_RASTER / 2);
   }

src/main/java/de/ph87/electro/circuit/part/parts/ConnectorDto.java

@@ -9,11 +9,11 @@ import lombok.ToString;
 @Getter
 @ToString
 @NoArgsConstructor
-public class ConnectorSubDto extends PartDto {
+public class ConnectorDto extends PartDto {
 
   private NodeDto node;
 
-  public ConnectorSubDto(final ConnectorSub part) {
+  public ConnectorDto(final Connector part) {
     node = new NodeDto(part.getNode());
   }
 

src/main/java/de/ph87/electro/circuit/part/parts/Light.java

@@ -1,10 +1,11 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Orientation;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.ToString;
 
 import java.awt.*;
@@ -24,13 +25,15 @@ public class Light extends Part {
 
   private static final Color BULB_OFF_COLOR = Color.DARK_GRAY;
 
+  private static final double DEFAULT_INNER_RESISTANCE = 15.0;
+
   private final Point size = new Point(RASTER, RASTER);
 
   private final Node a;
 
   private final Node b;
 
-  private double resistance = 15;
+  private final Resistance innerResistance;
 
   private double maxVoltage = 3;
 
@@ -40,19 +43,20 @@ public class Light extends Part {
 
   private Color color = BULB_OFF_COLOR;
 
-  public Light(final Point position) {
+  public Light(@NonNull final Point position) {
     super("Licht", position);
     a = addNode("A", P10, P50);
     b = addNode("B", P90, P50);
+    innerResistance = new Resistance(a, b, DEFAULT_INNER_RESISTANCE);
   }
 
-  public Light(final LightDto dto) {
+  public Light(@NonNull final LightDto dto) {
     super(dto);
     a = addNode(dto.getB(), P10, P50);
     b = addNode(dto.getA(), P90, P50);
-    resistance = dto.getResistance();
     maxVoltage = dto.getMaxVoltage();
     defect = dto.isDefect();
+    innerResistance = new Resistance(a, b, dto.getResistance());
   }
 
   @Override
@@ -73,7 +77,7 @@ public class Light extends Part {
   }
 
   @Override
-  protected void _render(final Graphics2D g) {
+  protected void _render(@NonNull final Graphics2D g) {
     drawLine(g, a, b, Color.BLACK, SYMBOL_STROKE);
     drawCircle(g, P50, P50, BULB_RADIUS, Color.BLACK, SYMBOL_STROKE, color, 0, 360);
 
@@ -83,7 +87,7 @@ public class Light extends Part {
   }
 
   @Override
-  protected void _labels(final Graphics2D g) {
+  protected void _labels(@NonNull final Graphics2D g) {
     final int x = RASTER / 2;
     final int y0;
     final int y1;
@@ -101,12 +105,13 @@ public class Light extends Part {
     }
   }
 
+  @NonNull
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     if (defect) {
       return Collections.emptyList();
     }
-    return List.of(new InnerConnection(a, b, resistance));
+    return List.of(innerResistance);
   }
 
 }

src/main/java/de/ph87/electro/circuit/part/parts/LightDto.java

@@ -26,7 +26,7 @@ public class LightDto extends PartDto {
     this.a = new NodeDto(light.getB());
     this.b = new NodeDto(light.getA());
     this.defect = light.isDefect();
-    this.resistance = light.getResistance();
+    this.resistance = light.getInnerResistance().getResistance();
     this.maxVoltage = light.getMaxVoltage();
   }
 

src/main/java/de/ph87/electro/circuit/part/parts/Meter.java

@@ -0,0 +1,108 @@
+package de.ph87.electro.circuit.part.parts;
+
+import de.ph87.electro.circuit.Resistance;
+import de.ph87.electro.circuit.part.Part;
+import de.ph87.electro.circuit.part.node.Node;
+import de.ph87.electro.common.RotationMatrix;
+import lombok.Getter;
+import lombok.NonNull;
+import lombok.Setter;
+import org.apache.commons.math3.linear.ArrayRealVector;
+import org.apache.commons.math3.linear.RealVector;
+
+import java.awt.*;
+import java.util.List;
+
+import static de.ph87.electro.CONFIG.*;
+import static de.ph87.electro.circuit.CircuitPainter.*;
+import static java.lang.Math.round;
+
+public abstract class Meter extends Part {
+
+  private static final int FINGER_LENGTH = P65;
+
+  private static final RealVector ANCHOR = new ArrayRealVector(new double[]{P50, P75});
+
+  private static final RealVector FINGER = new ArrayRealVector(new double[]{FINGER_LENGTH - P05, 0});
+
+  private static final Stroke SCALE_STROKE = new BasicStroke(1);
+
+  private static final double DEGREES_RANGE = 90;
+
+  public final MeterType type;
+
+  @Getter
+  public final Point size = new Point(RASTER, RASTER);
+
+  public final Node a;
+
+  public final Node b;
+
+  public final String unit;
+
+  public final Resistance innerResistance;
+
+  @Setter
+  public double min;
+
+  @Setter
+  public double max;
+
+  public Meter(@NonNull MeterType type, @NonNull final Point position) {
+    super(type.name, position);
+    this.type = type;
+    this.unit = type.unit;
+    this.min = type.min;
+    this.max = type.max;
+    a = addNode("A", P10, P90);
+    b = addNode("B", P90, P90);
+    innerResistance = new Resistance(a, b, type.resistance);
+  }
+
+  public Meter(@NonNull final MeterDto dto) {
+    super(dto);
+    this.type = dto.getType();
+    this.unit = dto.getUnit();
+    this.min = dto.getMin();
+    this.max = dto.getMax();
+    a = addNode("A", P10, P90);
+    b = addNode("B", P90, P90);
+    innerResistance = new Resistance(a, b, dto.getResistance());
+  }
+
+  @NonNull
+  public static Point P(@NonNull final RealVector anchor) {
+    return new Point((int) round(anchor.getEntry(0)), (int) round(anchor.getEntry(1)));
+  }
+
+  public abstract double getValue();
+
+  @Override
+  public void _render(@NonNull final Graphics2D g) {
+    // scale
+    final double DEGREES_OFFSET = 45;
+    drawCircle(g, P(ANCHOR), FINGER_LENGTH, DEGREES_OFFSET, DEGREES_RANGE, Color.BLACK, SCALE_STROKE, Color.white);
+
+    // finger
+    final double ratio = (getValue() - min) / (max - min);
+    final double degrees = -(DEGREES_OFFSET + DEGREES_RANGE * (1 - ratio));
+    final RealVector finger = RotationMatrix.rotateDegrees(FINGER, degrees);
+    final RealVector tip = ANCHOR.add(finger);
+    drawLine(g, P(ANCHOR), P(tip), Color.BLACK, SCALE_STROKE);
+
+    // anchor
+    drawCircle(g, P(ANCHOR), P03, 0, 360, null, null, Color.black);
+  }
+
+  @Override
+  protected void _labels(@NonNull final Graphics2D g) {
+    drawText(g, LABEL_FONT, "%.2f %s".formatted(getValue(), unit), P50, P90, Color.BLACK, orientation);
+  }
+
+  @NonNull
+  @Override
+  public List<Resistance> getResistances() {
+    return List.of(innerResistance);
+  }
+
+}

src/main/java/de/ph87/electro/circuit/part/parts/MeterDto.java

@@ -0,0 +1,41 @@
+package de.ph87.electro.circuit.part.parts;
+
+import de.ph87.electro.circuit.part.PartDto;
+import de.ph87.electro.circuit.part.node.NodeDto;
+import lombok.*;
+
+@Getter
+@ToString
+@NoArgsConstructor
+public class MeterDto extends PartDto {
+
+  private MeterType type;
+
+  private NodeDto a;
+
+  private NodeDto b;
+
+  @Setter
+  private double min;
+
+  @Setter
+  private double max;
+
+  @NonNull
+  @Setter
+  private String unit;
+
+  @Setter
+  private double resistance;
+
+  public MeterDto(@NonNull final Meter meter) {
+    type = meter.type;
+    a = new NodeDto(meter.a);
+    b = new NodeDto(meter.b);
+    min = meter.min;
+    max = meter.max;
+    unit = meter.unit;
+    resistance = meter.innerResistance.getResistance();
+  }
+
+}

src/main/java/de/ph87/electro/circuit/part/parts/MeterType.java

@@ -0,0 +1,28 @@
+package de.ph87.electro.circuit.part.parts;
+
+import static de.ph87.electro.CONFIG.RESISTANCE_MAX;
+import static de.ph87.electro.CONFIG.RESISTANCE_MIN;
+
+public enum MeterType {
+  VOLT("Voltmeter", "V", -3, 3, RESISTANCE_MAX),
+  AMPERE("Amperemeter", "A", -1, 1, RESISTANCE_MIN);
+
+  public final String name;
+
+  public final String unit;
+
+  public final double min;
+
+  public final double max;
+
+  public final double resistance;
+
+  MeterType(final String name, final String unit, final double min, final double max, final double resistance) {
+    this.name = name;
+    this.unit = unit;
+    this.min = min;
+    this.max = max;
+    this.resistance = resistance;
+  }
+
+}

src/main/java/de/ph87/electro/circuit/part/parts/Poti.java

@@ -1,6 +1,6 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
@@ -30,6 +30,10 @@ public class Poti extends Part {
 
   private final Point size = new Point(RASTER, RASTER);
 
+  private final Resistance resistance0;
+
+  private final Resistance resistance1;
+
   @Setter
   private double resistance = 10;
 
@@ -40,6 +44,8 @@ public class Poti extends Part {
     common = addNode("C", P10, P50);
     middle = addNode("M", P50, P10);
     end = addNode("E", P90, P50);
+    resistance0 = new Resistance(common, middle, max(RESISTANCE_MIN, resistance * ratio));
+    resistance1 = new Resistance(middle, end, max(RESISTANCE_MIN, resistance * (1 - ratio)));
   }
 
   public Poti(@NonNull final PotiDto dto) {
@@ -47,6 +53,8 @@ public class Poti extends Part {
     common = addNode(dto.getCommon().getName(), P10, P50);
     middle = addNode(dto.getMiddle().getName(), P50, P10);
     end = addNode(dto.getEnd().getName(), P90, P50);
+    resistance0 = new Resistance(common, middle, max(RESISTANCE_MIN, resistance * ratio));
+    resistance1 = new Resistance(middle, end, max(RESISTANCE_MIN, resistance * (1 - ratio)));
   }
 
   @Override
@@ -56,6 +64,8 @@ public class Poti extends Part {
     } else {
       ratio += 0.1;
     }
+    resistance0.setResistance(max(RESISTANCE_MIN, resistance * ratio));
+    resistance1.setResistance(max(RESISTANCE_MIN, resistance * (1 - ratio)));
   }
 
   @Override
@@ -63,11 +73,12 @@ public class Poti extends Part {
     drawText(g, LABEL_FONT, "%3.0f%%".formatted(ratio * 100), P50, P50, Color.BLACK, orientation);
   }
 
+  @NonNull
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     return List.of(
-      new InnerConnection(common, middle, max(NO_RESISTANCE, resistance * ratio)),
+      resistance0,
-      new InnerConnection(middle, end, max(NO_RESISTANCE, resistance * (1 - ratio)))
+      resistance1
     );
   }
 

src/main/java/de/ph87/electro/circuit/part/parts/Switch1x1.java

@@ -1,12 +1,14 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.ToString;
 
 import java.awt.*;
+import java.util.Collections;
 import java.util.List;
 
 import static de.ph87.electro.CONFIG.*;
@@ -24,22 +26,26 @@ public class Switch1x1 extends Part {
 
   private final Node output;
 
+  private final Resistance innerResistance;
+
   private boolean state = false;
 
-  public Switch1x1(final Point position) {
+  public Switch1x1(@NonNull final Point position) {
     super("Ausschalter", position);
     common = addNode("C", P10, P50);
     output = addNode("O", P90, P50);
+    innerResistance = new Resistance(common, output, RESISTANCE_MIN);
   }
 
-  public Switch1x1(final Switch1x1Dto dto) {
+  public Switch1x1(@NonNull final Switch1x1Dto dto) {
     super(dto);
     common = addNode(dto.getCommon(), P10, P50);
     output = addNode(dto.getOutput(), P90, P50);
     state = dto.isState();
+    innerResistance = new Resistance(common, output, RESISTANCE_MIN);
   }
 
-  public void setState(final boolean state) {
+  public void setState(@NonNull final boolean state) {
     this.state = state;
   }
 
@@ -49,7 +55,7 @@ public class Switch1x1 extends Part {
   }
 
   @Override
-  protected void _render(final Graphics2D g) {
+  protected void _render(@NonNull final Graphics2D g) {
     if (!state) {
       drawLine(g, common, END, common.getColor(), SWITCH_STROKE);
     } else {
@@ -57,12 +63,13 @@ public class Switch1x1 extends Part {
     }
   }
 
+  @NonNull
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     if (state) {
-      return List.of(new InnerConnection(common, output, NO_RESISTANCE));
+      return List.of(innerResistance);
     }
-    return super.getInnerConnections();
+    return Collections.emptyList();
   }
 
 }

src/main/java/de/ph87/electro/circuit/part/parts/Switch1x2.java

@@ -1,9 +1,10 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.ToString;
 
 import java.awt.*;
@@ -24,24 +25,32 @@ public class Switch1x2 extends Part {
 
   private final Node output1;
 
+  private final Resistance innerResistance0;
+
+  private final Resistance innerResistance1;
+
   private boolean state = false;
 
-  public Switch1x2(final Point position) {
+  public Switch1x2(@NonNull final Point position) {
     super("Wechselschalter", position);
     common = addNode("C", P10, P50);
     output0 = addNode("O0", P90, P25);
     output1 = addNode("O1", P90, P75);
+    innerResistance0 = new Resistance(common, output0, RESISTANCE_MIN);
+    innerResistance1 = new Resistance(common, output1, RESISTANCE_MIN);
   }
 
-  public Switch1x2(final Switch1x2Dto dto) {
+  public Switch1x2(@NonNull final Switch1x2Dto dto) {
     super(dto);
     common = addNode(dto.getCommon(), P10, P50);
     output0 = addNode(dto.getOutput0(), P90, P25);
     output1 = addNode(dto.getOutput1(), P90, P75);
     state = dto.isState();
+    innerResistance0 = new Resistance(common, output0, RESISTANCE_MIN);
+    innerResistance1 = new Resistance(common, output1, RESISTANCE_MIN);
   }
 
-  public void setState(final boolean state) {
+  public void setState(@NonNull final boolean state) {
     this.state = state;
   }
 
@@ -51,7 +60,7 @@ public class Switch1x2 extends Part {
   }
 
   @Override
-  protected void _render(final Graphics2D g) {
+  protected void _render(@NonNull final Graphics2D g) {
     if (!state) {
       drawLine(g, common, output0, common.getColor(), SWITCH_STROKE);
     } else {
@@ -60,11 +69,11 @@ public class Switch1x2 extends Part {
   }
 
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     if (state) {
-      return List.of(new InnerConnection(common, output1, NO_RESISTANCE));
+      return List.of(innerResistance1);
     }
-    return List.of(new InnerConnection(common, output0, NO_RESISTANCE));
+    return List.of(innerResistance0);
   }
 
 }

src/main/java/de/ph87/electro/circuit/part/parts/SwitchCross.java

@@ -1,9 +1,10 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.Part;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.ToString;
 
 import java.awt.*;
@@ -26,6 +27,14 @@ public class SwitchCross extends Part {
 
   private final Node output1;
 
+  private final Resistance innerResistance01;
+
+  private final Resistance innerResistance10;
+
+  private final Resistance innerResistance00;
+
+  private final Resistance innerResistance11;
+
   private boolean state = false;
 
   public SwitchCross(final Point position) {
@@ -34,6 +43,10 @@ public class SwitchCross extends Part {
     common1 = addNode("C1", P10, P75);
     output0 = addNode("O0", P90, P25);
     output1 = addNode("O1", P90, P75);
+    innerResistance01 = new Resistance(common0, output1, RESISTANCE_MIN);
+    innerResistance10 = new Resistance(common1, output0, RESISTANCE_MIN);
+    innerResistance00 = new Resistance(common0, output0, RESISTANCE_MIN);
+    innerResistance11 = new Resistance(common1, output1, RESISTANCE_MIN);
   }
 
   public SwitchCross(final SwitchCrossDto dto) {
@@ -43,6 +56,10 @@ public class SwitchCross extends Part {
     output0 = addNode(dto.getOutput0(), P90, P25);
     output1 = addNode(dto.getOutput1(), P90, P75);
     state = dto.isState();
+    innerResistance01 = new Resistance(common0, output1, RESISTANCE_MIN);
+    innerResistance10 = new Resistance(common1, output0, RESISTANCE_MIN);
+    innerResistance00 = new Resistance(common0, output0, RESISTANCE_MIN);
+    innerResistance11 = new Resistance(common1, output1, RESISTANCE_MIN);
   }
 
   public void setState(final boolean state) {
@@ -65,17 +82,18 @@ public class SwitchCross extends Part {
     }
   }
 
+  @NonNull
   @Override
-  public List<InnerConnection> getInnerConnections() {
+  public List<Resistance> getResistances() {
     if (state) {
       return List.of(
-        new InnerConnection(common0, output1, NO_RESISTANCE),
+        innerResistance01,
-        new InnerConnection(common1, output0, NO_RESISTANCE)
+        innerResistance10
       );
     }
     return List.of(
-      new InnerConnection(common0, output0, NO_RESISTANCE),
+      innerResistance00,
-      new InnerConnection(common1, output1, NO_RESISTANCE)
+      innerResistance11
     );
   }
 

src/main/java/de/ph87/electro/circuit/part/parts/Voltmeter.java

@@ -1,98 +1,24 @@
 package de.ph87.electro.circuit.part.parts;
 
-import de.ph87.electro.circuit.part.InnerConnection;
-import de.ph87.electro.circuit.part.Part;
-import de.ph87.electro.circuit.part.PartDto;
-import de.ph87.electro.circuit.part.node.Node;
-import de.ph87.electro.common.RotationMatrix;
 import lombok.Getter;
-import lombok.Setter;
+import lombok.NonNull;
-import org.apache.commons.math3.linear.ArrayRealVector;
-import org.apache.commons.math3.linear.RealVector;
 
 import java.awt.*;
-import java.util.List;
-
-import static de.ph87.electro.CONFIG.*;
-import static de.ph87.electro.circuit.CircuitPainter.*;
-import static java.lang.Math.round;
 
 @Getter
-public class Voltmeter extends Part {
+public class Voltmeter extends Meter {
 
-  private static final int FINGER_LENGTH = P65;
+  public Voltmeter(@NonNull final Point position) {
-
+    super(MeterType.VOLT, position);
-  private static final RealVector ANCHOR = new ArrayRealVector(new double[]{P50, P75});
-
-  private static final RealVector FINGER = new ArrayRealVector(new double[]{FINGER_LENGTH - P05, 0});
-
-  private static final Stroke SCALE_STROKE = new BasicStroke(1);
-
-  private static final double DEGREES_RANGE = 90;
-
-  private final Point size = new Point(RASTER, RASTER);
-
-  private final Node a;
-
-  private final Node b;
-
-  double x = 0;
-
-  @Setter
-  private double min = -3;
-
-  @Setter
-  private double max = 3;
-
-  private RealVector tip;
-
-  public Voltmeter(final Point position) {
-    super("Voltmeter", position);
-    a = addNode("A", P10, P90);
-    b = addNode("B", P90, P90);
-    postCalculate(); // TODO remove
   }
 
-  public Voltmeter(final PartDto dto) {
+  public Voltmeter(@NonNull final MeterDto dto) {
     super(dto);
-    a = addNode("A", P10, P90);
-    b = addNode("B", P90, P90);
-    postCalculate(); // TODO remove
-  }
-
-  public static Point P(final RealVector anchor) {
-    return new Point((int) round(anchor.getEntry(0)), (int) round(anchor.getEntry(1)));
   }
 
   @Override
-  public void _render(final Graphics2D g) {
+  public double getValue() {
-    // scale
-    final double DEGREES_OFFSET = 45;
-    drawCircle(g, P(ANCHOR), FINGER_LENGTH, DEGREES_OFFSET, DEGREES_RANGE, Color.BLACK, SCALE_STROKE, Color.white);
-
-    // finger
-    final double ratio = (getVoltage() - min) / (max - min);
-    final double degrees = -(DEGREES_OFFSET + DEGREES_RANGE * (1 - ratio));
-    final RealVector finger = RotationMatrix.rotateDegrees(FINGER, degrees);
-    tip = ANCHOR.add(finger);
-    drawLine(g, P(ANCHOR), P(tip), Color.BLACK, SCALE_STROKE);
-
-    // anchor
-    drawCircle(g, P(ANCHOR), P03, 0, 360, null, null, Color.black);
-  }
-
-  private double getVoltage() {
     return !Double.isNaN(b.getVoltage()) && !Double.isNaN(a.getVoltage()) ? b.getVoltage() - a.getVoltage() : 0.0;
   }
 
-  @Override
-  protected void _labels(final Graphics2D g) {
-    drawText(g, LABEL_FONT, "%.2f V".formatted(getVoltage()), P50, P90, Color.BLACK, orientation);
-  }
-
-  @Override
-  public List<InnerConnection> getInnerConnections() {
-    return List.of(new InnerConnection(a, b, MAX_RESISTANCE));
-  }
-
 }

src/main/java/de/ph87/electro/circuit/part/parts/VoltmeterDto.java

@@ -1,32 +0,0 @@
-package de.ph87.electro.circuit.part.parts;
-
-import de.ph87.electro.circuit.part.PartDto;
-import de.ph87.electro.circuit.part.node.NodeDto;
-import lombok.Getter;
-import lombok.NoArgsConstructor;
-import lombok.Setter;
-import lombok.ToString;
-
-@Getter
-@ToString
-@NoArgsConstructor
-public class VoltmeterDto extends PartDto {
-
-  private NodeDto a;
-
-  private NodeDto b;
-
-  @Setter
-  private double min;
-
-  @Setter
-  private double max;
-
-  public VoltmeterDto(final Voltmeter voltmeter) {
-    a = new NodeDto(voltmeter.getA());
-    b = new NodeDto(voltmeter.getB());
-    min = voltmeter.getMin();
-    max = voltmeter.getMax();
-  }
-
-}

src/main/java/de/ph87/electro/circuit/wire/Wire.java

@@ -1,5 +1,6 @@
 package de.ph87.electro.circuit.wire;
 
+import de.ph87.electro.circuit.Resistance;
 import de.ph87.electro.circuit.part.node.Node;
 import lombok.Getter;
 import lombok.NonNull;
@@ -14,23 +15,13 @@ import static java.lang.Math.*;
 
 @Getter
 @ToString
-public class Wire {
+public class Wire extends Resistance {
-
-  @NonNull
-  private final Node a;
-
-  @NonNull
-  private final Node b;
-
-  @Setter
-  private double current = Double.NaN;
 
   @Setter
   private boolean ghost = false;
 
   public Wire(@NonNull final Node a, @NonNull final Node b) {
-    this.a = a;
+    super(a, b, RESISTANCE_MIN);
-    this.b = b;
     a.getWires().add(this);
     b.getWires().add(this);
   }
@@ -56,19 +47,6 @@ public class Wire {
     return distance <= WIRE_HOVER_STROKE_BACK.getLineWidth();
   }
 
-  public Node getOpposite(final Node node) {
-    if (a == node) {
-      return b;
-    } else if (b == node) {
-      return a;
-    }
-    throw new RuntimeException();
-  }
-
-  public double getCurrent(final Node node) {
-    return node == a ? current : -current;
-  }
-
   public void draw(final Graphics2D g) {
     final Color color = ghost ? new Color(a.getColor().getRed(), a.getColor().getGreen(), a.getColor().getBlue(), 64) : a.getColor();
 

src/main/java/de/ph87/electro/sidebar/Sidebar.java

@@ -29,6 +29,7 @@ public class Sidebar extends JPanel {
     addPart(new SwitchCross(ZERO));
     addPart(new Poti(ZERO));
     addPart(new Voltmeter(ZERO));
+    addPart(new Amperemeter(ZERO));
   }
 
   @SuppressWarnings("UnusedReturnValue")