132 A DIRECT COMPARISON OF ELECTROSTATIC 



key, so thut. when the key was pressed, the current went first through the 

 secondary ooil of the galvanometer, consisting of thirty windings of thick wire, 

 then through the fixed coil, then to the suspension wire, and so through the 

 two suspended coils to the brass frame of the torsion-balance and the suspended 

 disk. A stout copper wire, well amalgamated, hanging from the centre of the 

 toreinii-lnliince into a cup of mercury, made metallic communication to the case, 

 to earth, and to the other electrode of the battery. 



When these arrangements had been made, the observer at the microscope, 

 when the suspended disk was stationary at zero, made simultaneous contact 

 with both batteries by means of the key. If the disk was attracted, the 

 great battery was the more powerful, and the micrometer was worked so as 

 to increase the distance of the disk. If the disk was repelled, the fixed disk 

 had to be moved nearer to the suspended disk, till a distance was found at 

 which, when the scale was at rest and at zero, no effect was produced by 

 the simultaneous action of the batteries. With the forces actually employed 

 the equilibrium of the scale at zero was unstable ; so that when the adjust- 

 ment was nearly perfect the force was always directed from zero, and contacts 

 had to be made as the scale was approaching zero, in such a way as to 

 bring it to rest, if possible, at zero. 



In the meantime the other observer at the galvanometer was taking 

 advantage of these contacts to alter the shunt S, till the effects of the two 

 currents on the galvanometer-needle balanced each other. 



When a satisfactory case of equilibrium had been observed simultaneously 

 at the galvanometer and at the torsion-balance, the micrometer-reading and the 

 resistance of the shunt were set down as the results of the experiment. 



The chief difficulties experienced arose from the want of constancy in the 

 batteries, the ratio of the currents varying very rapidly after first making 

 contact. I think that by increasing considerably the resistance of the great 

 battery-circuit, the current could be made more uniform. 



When a sufficient number of experiments on equilibrium had been made, 

 a current was made to pass through the secondary coil of the galvanometer, 

 and was then divided between a shunt of 31 units BA and the primary mil 

 <>f the galvanometer with a resistance & added. S" was then varied till the 

 needle was in equilibrium. In this way the magnetic effects of the two coils 

 were compared. 



The resistance of the galvanometer and of all the coils were tested by 



