144 REPORT—1885. 
Schiller’s Hxperiments. 
The first experiment which we shall discuss is one made by Schiller, 
and described by him in Poggendorf’s Annalen, vol. clix. pp. 456, 537 ; 
it was intended to test the potential theories of Neumann and Helm- 
holtz. We saw that, according to these theories, in an unclosed circuit 
there are, in addition to the forces due to the elements of current, and 
which are expressed by Ampére’s law, forces arising from the discon- 
tinuity of the currents at the ends of the circuit. If we have an end of a 
circuit where the current stops, and the electricity accumulates at the 
rate de/dt, it will exert on an element of current of length ds traversed 
by a current of intensity i a force tending to the end and equal to 
where 0 is the angle between the element of current and the radius drawn 
to it from the end. If we calculate from this expression the couple pro- 
duced by an end on an endless solenoid, or on what is practically the 
same thing, a ring magnet, we shall find that the couple tending to turn 
the ring about an axis in its own place will not vanish, while the couple 
arising from the forces given by Ampére’s law will. Thus if the ring 
rotates, as it should according to the potential theory, it must be from 
the action of the end. 
In Schiller’s experiment the end of the current was the end of wire 
connected with a Holtz machine. This was placed near to a ring magnet 
which was suspended by a long cocoon fibre; the magnet was protected 
from electrostatic influences by being enclosed in a metal box connected 
with the earth. Schiller determined the intensity of magnetisation of 
the ring magnet and the quantity of electricity passing through the 
point, and he calculated that if the potential theory were true, he ought 
to get a deflection of the magnet of about 27 scale divisions, instead of 
which there was no perceptible deflection. 
This experiment shows conclusively that the potential theory is wrong 
if we neglect altogether the action of the dielectric, and assume the cur- 
rent to stop at the end of the wire. If, however, we take the dielectric 
into account, the experiment tells us nothing as to whether Maxwell’s 
theory or the more general one is true; for since the current from the 
Holtz machine is steady, as much electricity flows out from the end of 
the wires as arrives there; and thus there is really no discontinuity in 
the current, the only difference being that before reaching the end the 
current is flowing through copper and after passing it through air. The 
condition of things at the end of the wire remains steady, and thus the 
quantities which we denoted by P and = vanish. 
The experiment might, however, be modified so as to be capable of 
distinguishing between the theories which take the dielectric into account, 
For suppose that, instead of letting the electricity escape through the 
point, we never let the potential at the end of the wire get so high as to 
allow the electricity to escape ; then if the wire is initially uncharged, the 
condition at the end will be changing whilst the wire is charging up, and 
thus = will have afinite value; so that if the magnet were sufficiently 
delicate and remained undeflected, whilst the point was surrounded by 
dielectrics of all kinds, it would show that Maxwell’s theory is correct. 
I have calculated the effect which would be produced on Schiller’s 
