386 Prof. W. C. Réntgen on the Electromagnetic 
Helmholtz has shown that none of the consequences of this _ 
assumption are opposed to the fundamental laws of mechanics, 
and that this assumption, together with the extended law of 
potential, affords a complete explanation of the phenomena 
presented by closed and by so-called open conductors. 
So far as I know, however, no direct experimental proof 
has been given of the correctness of the assumption of Fara- 
day and Maxwell; and I have for several years had the inten- 
tion to fill this gap when the opportunity presented itself. 
After many fruitless attempts I have at last been successful in 
finding a method which yields practical and decisive results, 
of which I venture here to give an account. A good insu- 
lating disk of ebonite, 0°5 centim. thick and 16 centim. dia- 
meter, was mounted upon a vertical axis, which by means of 
a string could be put into rapid rotation (120 to 150 revolu- 
tions per second) in a horizontal plane. Beneath the disk 
and parallel with it was placed a perforated glass plate of 17°5 
centim. diameter, provided with two half-ring coatings of 
tinfoil: the inner radius of the rings was 2°25 centim. and the 
outer radius 7 centim.; the portion cut out between the two 
half-rings was 1:4 centim. wide. Above the ebonite disk also 
was fixed a second horizontal glass plate of 21°5 centim. dia- 
meter, which was completely coated with tinfoil. The tinfoil 
coatings of the two glass plates faced the ebonite disk, and 
were at a distance from it of about 0-1 centim.; the coating 
of the upper plate was permanently connected to earth. Hither 
of the half-rings could be placed in connection with the 
inner coating of a large Leyden jar, so that the one became 
positively electrified, and the other at the same time nega- 
tively. A commutator permitted the electrification to be 
changed. 
It will now be understood that the dielectric polarization 
produced in the rotating ebonite disk by the electrification of 
the tinfoil coatings changes its sign at the point where the 
interval between the coatings occurred. Upon the one half 
of the disk (say the front half) the particles moved from the 
positive towards the negative half-ring, and a displacement of 
positive electricity took place while they passed from the one 
half-ring to the other, which would have a vertical component 
tending downwards. At the same time on the other (hinder) 
half of the disk, there would be a vertical component tending 
upwards. These displacements lasted as long as the disk 
rotated with unaltered electrification; and they must there- 
fore, according to the theory of Faraday and Maxwell, pro- 
duce the same electromagnetic effect as continuous currents, 
which, for the direction of rotation assumed, would circulate 
downwards in the front half of the disk, and upwards in the 
