418 MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
light, which requires that the square of the refractive index should be equal to the 
dielectric capacity, multiplied by the magnetic permeability. 
By March, 1877, the method of work had been sketched out, and most of the 
instruments ordered. 
The greater portion of the expense of the investigation has been met by grants from 
the Government Fund of £4,000. 
The general outline of the method, which is, with the exception of the first idea of 
the 5-plate balance,”' entirely due to Professor Maxwell, is as follows :— 
Metal plates, some connected with the electrometer and some with the source of 
electrification, are so placed that, when there is only air between them, the potentials 
of the two pairs of quadrants are equal and the needle remains at zero, however the 
strength of the electrification may vary. 
On placing one of the dielectrics under examination between two of the plates a 
stronger action takes place between these two plates, and the needle is deflected. The 
plates are now moved farther apart by a screw so as to reduce the action until the 
needle returns to zero. 
From the distance which the plates have to be separated, and the thickness of the 
dielectric, the specific inductive capacity can be calculated. 
The great difficulty in all experiments in specific inductive capacity is to make the 
experiments quickly enough. If the electric stress is continued in the same direction 
for auy appreciable time, a permanent strain is produced, and the apparent specific 
inductive capacity differs from the true one. 
To overcome this difficulty Professor Maxwell arranged that the electrification 
should be constantly reversed. It was found possible to reverse it 12,000 times in a 
second, and so to eliminate all phenomena of “residual charge.” 
How, in spite of the constant reversal, it was arranged that the deflection, when the 
balance was not established, should be steady, and depend only on the position of the 
plates, and not on the direction of the electrification, will be seen later on in the paper. 
Description of instruments employed. 
For producing a rapidly reversed electrification of high potential, a coil, wheel-break, 
secondary reversing engine, and batteries were used. 
The coil was one of Alps’ induction coils, on which is wound 22 miles of secondary 
wire, and which, with a suitable battery and break, is capable of giving a 17-inch spark 
in air. 
The wheel-break consists of a small magnetic engine the fly-wheel of which is of 
brass, and about 2| inches in diameter. In the circumference of the wheel 60 slots 
are cut and filled with ebonite. A light spring faced with platinum presses on the 
edge of the wheel with a force regulated by a capstan screw. 
* Dne, I believe, to Sir William Thomson. 
