proposed fix for incorrect EMF in State wrapper, #149

js/transformer/model/Transformer.ts

@@ -19,6 +19,7 @@ import RangeWithValue from '../../../../dot/js/RangeWithValue.js';
 import PickRequired from '../../../../phet-core/js/types/PickRequired.js';
 import optionize from '../../../../phet-core/js/optionize.js';
 import ConstantDtClock from '../../common/model/ConstantDtClock.js';
+import isSettingPhetioStateProperty from '../../../../tandem/js/isSettingPhetioStateProperty.js';
 
 type SelfOptions = {
   electromagnetPosition: Vector2;
@@ -73,9 +74,11 @@ export default class Transformer extends PhetioObject {
     // Workaround for https://github.com/phetsims/faradays-electromagnetic-lab/issues/92, to ignore the EMF induced
     // by switching power supplies. Step the pickup coil twice, so that there is effectively no induced EMF.
     this.electromagnet.currentSourceProperty.lazyLink( () => {
-      this.pickupCoil.step( ConstantDtClock.DT ); // EMF may be induced by changing from oldCurrentSource to newCurrentSource.
-      this.pickupCoil.step( ConstantDtClock.DT ); // No EMF is induced because there is no flux change in newCurrentSource.
-      assert && assert( this.pickupCoil.emfProperty.value === 0, `unexpected emfProperty.value: ${this.pickupCoil.emfProperty.value}` );
+      if ( !isSettingPhetioStateProperty.value ) {
+        this.pickupCoil.step( ConstantDtClock.DT ); // EMF may be induced by changing from oldCurrentSource to newCurrentSource.
+        this.pickupCoil.step( ConstantDtClock.DT ); // No EMF is induced because there is no flux change in newCurrentSource.
+        assert && assert( this.pickupCoil.emfProperty.value === 0, `unexpected emfProperty.value: ${this.pickupCoil.emfProperty.value}` );
+      }
     } );
   }