650 
ME. CLEEK MAXWELL ON A DIEECT COMPAEISON 
In this expression the only quantities which must be determined in absolute measure 
are the resistances. The other quantities which must be measured are the ratios of the 
radius of the disk to its distance from the fixed disk, and the ratio of the radius of the 
coils to the distance between them. These ratios and the number of windings in the 
coils are of course abstract numbers. 
In the experiments, 
w=144 ri = 121 
a— 2-0977 inches. a' = T934 inch. 
To determine a', the circumference of every layer of the coils was measured with 
watch-spring, the thickness of which was -008 inch. 
One turn of the micrometer-screw was found by Mr. Hockin' to be equal to *0202 inch. 
If m is the micrometer-reading in terms of the screw, 
b=m- 12-70, V=m+ 26*31. 
In terms of the micrometer measure we have for a and a', 
a= 103-85 turns, a!= 95-75 turns. 
The resistances were determined by Mr. Hockin as follows : 
E = 1 102 000 Ohms. 
G= 46 220 „ 
The experiments were made for two days, using a small battery charged with bichro- 
mate of potash. The current due to this battery was found to diminish so rapidly that 
a set of Grove’s cells was used on the third day, which was found to be more constant 
than the great battery. A proper combination of the two batteries would perhaps pro- 
duce a current which would diminish according to the same law as that of the great 
battery. Another difficulty arose from the fact that when the connexions were made, 
but before the key was pressed, if the micrometer was touched by the hand the disk 
was attracted. This I have not been able satisfactorily to account for, except by leak- 
age of electricity from the great battery through the floor. When the micrometer was not 
touched, the disk remained at its proper zero. In certain experiments I kept my hand 
always on the micrometer in order to be able to adjust it more accurately. These expe- 
riments gave a value of v much too small, on account of the additional attraction. When 
I discovered the attraction, I took care to make the observations without touching the 
micrometer, and took advantage of the attraction to check the oscillations of the disk. 
The experiments in which these precautions were taken agree together as well as I could 
expect, and lead me to think that, with the experience I have acquired, still better re- 
sults might be obtained by the same method. It must be borne in mind that none of 
the results were calculated till after the conclusion of all the experiments, and that the 
rejected experiments were condemned on account of errors observed while they were 
being made. 
Any leakage arising from want of insulation of the fixed disk would introduce no 
