19/02/2004 23:32:13
According to my finite element analysis of the force on an induced current loop inside an alternating current loop there is a small central force on the inner current loop. This experiment is to test the result of this calculation.
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The diagram on the right shows the transformer winding with a small shorted loop of copper wire suspended by a cotton thread. The expected force is shown by the arrow. On power there was no sign of any deflection. |
The mains power was applied to a transformer coil with a series resistor of 470 ohm to limit the current to about 400 mA. This alternating current flowed in the winding for about one second. During this time I could see no obvious deflection of the shorted loop.
The central force on a current loop inside a current loop is small or non existent.
I may try again with a higher power.
I am planning the same experiment again using the same equipment above but with a Stanton A43 type microgram balance to measure the central force.
This is the apparatus at present. I need a mains transformer for powering the lamp house of the microgram balance as I need the illuminated vernier scale to show any deflection and to weigh the force.
It is an old second hand balance that was scrapped by its previous user and the mechanical weighing arm has balance masses missing, the mains psu is missing, the levelling device is missing and the scale pans are missing.
However the essential balance arm and agate bearings are intact. So also is the optical lever for measuring balance and to weigh small masses.
My result will be in grams force rather than newtons.
Using the results from the table it is likely that the force will be of the order of 1E-3 newtons or about 10 milligrams and so will be detectable with this apparatus.
I hope to complete this part of my investigation by the end of next week (7-2-2004)
5-2-2004: The balance was equipped with a 6v mains transformer to power the optical lever and scale. I balanced the arm with no current then switched on an alternating current of 400mA the are registered no deflection then drifted down the scale. I found that the coils were getting hot so deduce that the drift was due to a rising current of air. According to the scale the balance could measure a force of 0.05 mg, this implies that if there was a force, then it was less than 0,05mg and since the predicted force was 10mg I can only come to the conclusion that my calculation was wrong and there is no central force on a shorted loop inside a solenoid carrying alternating current.
In this test I used another pair of coils, see below:
The shorted loop was about 5mm in diameter and the transformer windings about 18mm square. The current of 400mA flowed in about 400 turns of wire making the magnetising force of about 160 Ampere-Turns. The shorted loop was hung approximately midway between the two coils and was hung so it was just underneath the top of the winding (in the space).
Note: In a subsequent test I found that:
The balance does not respond when small masses are added to either dash-pot nor does changing the counterbalance weight by a small step change the scale reading.
I conclude that the balance is sticky and cannot measure small forces (<10mg).
This invalidates the measurements.
The balance had had its spirit level removed by vandals at some time, so I could not level the balance, I was advised that this would cause the balance to stick. I purchased to two axis spirit level locally and found the balance was out so I levelled it. This cleared the stiffness.
I repeated the measurement using the coil pair with 300mA flowing. With no current the deflection was 49mg and with the current of 300mA the deflection was 48.8mg this deflection returned to 49mg on stopping the current implying a deflection of 0.2 mg due to the current. So there is a central force on a shorted loop inside a solenoid carrying alternating current.
Further measurements with extra precautions against draught and vibration confirmed this result see pictures to see experimental arrangement. I put the little shorted loop to the bottom of the coil and, as predicted, the deflection was opposite.
Disclaimer: The deflection involved was at the limit of sensitivity of the balance and there was some random movement of the balance arm. I cannot be absolutely certain that the observed deflection was due to the predicted force. I just chose a period of no movement when the arm had settled down to make my measurements. In order to switch on the current I had to press a switch and this could an involuntary shift in my body weight. This could have moved the balance. I tried to eliminate this with multiple measurements and also by altering the current to see if the deflection altered. Both these criteria indicated the reality of the effect.
I would like another person to try with a more stable arrangement.
See Inductance See Diagram see also Tokomak and experiment 1, experiment 2, proposed experiment.
Chris.
19/02/2004
