Experiment to find the force
On a shorted loop in a current loop
carrying alternating current.


Today I set up two experiments to find the force on a shorted loop in the field due to an alternating current in (1) single turn of wire carrying alternating current and (2) the force on the same shorted loop in a solenoid carrying alternating current.

In both cases I found no evidence of any force.  If there is one then its magnitude is smaller than my apparatus can detect..

1. Single Turn

Diagram showing balance arrangement

a) An alternating current of about one amp was passed through a single turn of wire 50 cm in diameter and was configured with a switch so that it could be turned on and off.  A shorted loop  about ten cm in diameter was suspended on a balance arm pivoted some way from the loop within the single turn but above the centre.  A counterbalance ensured stability.

When the current was applied there was no visible movement of the shorted loop.

2 Solenoid

  Diagram similar to above.

Similarly to a) but with the shorted loop suspended near the mid point of the solenoid but above the centre axis.

When the current was applied there was no visible movement of the loop.



My calculation predicting a force towards the centre under these conditions was wrong and if there is a force then it is smaller than my simple apparatus can detect.

I have repeated this experiment with a demounted mains transformer with two coils mounted with a gap between them.  The force on a small loop (0.5 cm dia) was measured with 400 mA 50 Hz current flowing, each winding had 392 turns making  784 turns altogether.  That makes 313 Ampere Turns since the coil was 6 cm long the magnetising force was 52266 Ampere-Turns per meter that makes 72.9 Webers/Meter squared.

The measured force was about 2 mg towards the centre.

This is about the same as predicted by calculation using an off axis loop inside a loop carrying alternating current.  See LCS download.

General view of arrangement, a sewing machine was the heaviest stable platform, I had to wait until 1 am for vibrations around me to cease.

View through side of balance

Balance with power supply variac and transformer.

This shows end view of transformer coils.

This shows detail of loop inside gap between transformer coil.