add gradientField query parameter for testing, #66

js/common/FELConstants.ts

@@ -204,7 +204,10 @@ const FELConstants = {
     'while the sign indicates the direction of flow.',
 
   // Diameter for charged particles (electrons and imaginary positive charges)
-  CHARGED_PARTICLE_DIAMETER: 9
+  CHARGED_PARTICLE_DIAMETER: 9,
+
+  // See FELQueryParameters.calibrationField
+  GRADIENT_FIELD_X_RANGE: new Range( 100, 650 )
 };
 
 faradaysElectromagneticLab.register( 'FELConstants', FELConstants );

js/common/FELQueryParameters.ts

@@ -9,6 +9,7 @@
 
 import logGlobal from '../../../phet-core/js/logGlobal.js';
 import faradaysElectromagneticLab from '../faradaysElectromagneticLab.js';
+import FELConstants from './FELConstants.js';
 
 export const MagneticUnitsValues = [ 'G', 'T' ] as const;
 export type MagneticUnits = ( typeof MagneticUnitsValues )[number];
@@ -78,6 +79,27 @@ const SCHEMA_MAP = {
   // FELScreenView base class that is shared by all screens.
   showFieldPositions: {
     type: 'flag'
+  },
+
+  // Ignores the magnet model and sets up a static gradient B-field, where all field vectors are (Bx,0), and Bx varies
+  // linearly over a range. The value for this query parameter is the range of Bx from positions x=100 to x=650.
+  // These positions will be noted by yellow vertical lines. By is always 0.
+  //
+  // This is useful for calibrating the sim, and verifying behavior of the pickup coil. This query parameter affects
+  // all screens, even though it is only useful in the Pickup Coil and Transformer screens.
+  //
+  // Examples:
+  // calibrationField=300,300 sets up a B-field that has Bx=300 everywhere.
+  // calibrationField=0,300 sets up a B-field that linearly increases from Bx=0 to Bx=300 from left to right.
+  //
+  gradientField: {
+    type: 'array',
+    elementSchema: {
+      type: 'number',
+      isValidValue: ( value: number ) => ( Number.isInteger( value ) && FELConstants.MAGNET_STRENGTH_RANGE.contains( value ) )
+    },
+    defaultValue: null,
+    isValidValue: ( array: null | number[] ) => ( array === null ) || ( array.length === 2 )
   }
 } as const;
 

js/common/model/Magnet.ts

@@ -19,6 +19,9 @@ import BooleanProperty from '../../../../axon/js/BooleanProperty.js';
 import TReadOnlyProperty from '../../../../axon/js/TReadOnlyProperty.js';
 import Bounds2 from '../../../../dot/js/Bounds2.js';
 import Dimension3 from '../../../../dot/js/Dimension3.js';
+import Utils from '../../../../dot/js/Utils.js';
+import FELQueryParameters from '../FELQueryParameters.js';
+import FELConstants from '../FELConstants.js';
 
 type SelfOptions = {
 
@@ -131,19 +134,32 @@ export default abstract class Magnet extends FELMovable {
 
     returnVector = returnVector || new Vector2( 0, 0 );
 
-    // Convert to the magnet's local coordinate frame, writes to this.reusablePosition.
-    this.globalToLocalPosition( position, this.reusablePosition );
+    if ( FELQueryParameters.gradientField ) {
 
-    // Get strength in the magnet's local coordinate frame, writes to returnVector.
-    this.getLocalFieldVector( this.reusablePosition, returnVector );
+      // Create a B-field whose Bx changes linearly from left to right, and whose By is always 0.
+      const minX = FELConstants.GRADIENT_FIELD_X_RANGE.min;
+      const maxX = FELConstants.GRADIENT_FIELD_X_RANGE.max;
+      const minBx = FELQueryParameters.gradientField[ 0 ];
+      const maxBx = FELQueryParameters.gradientField[ 1 ];
+      const Bx = Utils.clamp( Utils.linear( minX, maxX, minBx, maxBx, position.x ), minBx, maxBx );
+      const By = 0;
+      returnVector.setXY( Bx, By );
+    }
+    else {
+      // Convert to the magnet's local coordinate frame, writes to this.reusablePosition.
+      this.globalToLocalPosition( position, this.reusablePosition );
+
+      // Get strength in the magnet's local coordinate frame, writes to returnVector.
+      this.getLocalFieldVector( this.reusablePosition, returnVector );
 
-    // Adjust the field vector to match the magnet's direction.
-    returnVector.rotate( this.rotationProperty.value );
+      // Adjust the field vector to match the magnet's direction.
+      returnVector.rotate( this.rotationProperty.value );
 
-    // Do not allow field vector magnitude to exceed magnet strength, due to small floating-point error.
-    // This was a problem in both the Java and HTML5 versions of the sim.
-    if ( returnVector.magnitude > this.strengthProperty.value ) {
-      returnVector.setMagnitude( this.strengthProperty.value );
+      // Do not allow field vector magnitude to exceed magnet strength, due to small floating-point error.
+      // This was a problem in both the Java and HTML5 versions of the sim.
+      if ( returnVector.magnitude > this.strengthProperty.value ) {
+        returnVector.setMagnitude( this.strengthProperty.value );
+      }
     }
     return returnVector;
   }

js/common/view/FELScreenView.ts

@@ -11,7 +11,7 @@ import ScreenView, { ScreenViewOptions } from '../../../../joist/js/ScreenView.j
 import ResetAllButton from '../../../../scenery-phet/js/buttons/ResetAllButton.js';
 import FELConstants from '../../common/FELConstants.js';
 import faradaysElectromagneticLab from '../../faradaysElectromagneticLab.js';
-import { Node } from '../../../../scenery/js/imports.js';
+import { Line, Node } from '../../../../scenery/js/imports.js';
 import Multilink from '../../../../axon/js/Multilink.js';
 import FieldNode from '../../common/view/FieldNode.js';
 import FieldMeterNode from '../../common/view/FieldMeterNode.js';
@@ -28,6 +28,7 @@ import Property from '../../../../axon/js/Property.js';
 import PickupCoilNode from './PickupCoilNode.js';
 import DerivedProperty from '../../../../axon/js/DerivedProperty.js';
 import isSettingPhetioStateProperty from '../../../../tandem/js/isSettingPhetioStateProperty.js';
+import FELQueryParameters from '../FELQueryParameters.js';
 
 type SelfOptions = {
 
@@ -126,6 +127,14 @@ export default class FELScreenView extends ScreenView {
           timeControlNode.bottom = visibleBounds.bottom - FELConstants.SCREEN_VIEW_Y_MARGIN;
         } );
     }
+
+    // Add vertical lines to show the range of x over which the gradient B-field extends.
+    if ( FELQueryParameters.gradientField ) {
+      const minX = FELConstants.GRADIENT_FIELD_X_RANGE.min;
+      const maxX = FELConstants.GRADIENT_FIELD_X_RANGE.max;
+      this.addChild( new Line( minX, this.layoutBounds.minY, minX, this.layoutBounds.maxY, { stroke: 'yellow' } ) );
+      this.addChild( new Line( maxX, this.layoutBounds.minY, maxX, this.layoutBounds.maxY, { stroke: 'yellow' } ) );
+    }
   }
 
   /**