648 
ME. CLEEK MAXWELL ON A DIRECT COMPAEISON 
of the suspended disk could be made to coincide with the plane of the guard-ring to the 
thousandth of an inch ; and the adjustment'when made continued very good from day to 
day, soft copper wire, stretched straight, not having the tendency to untwist gradually 
which I have observed in steel wire. The weight of the torsion piece was about 1 lb. 
3 oz., and the time of a double oscillation about fourteen seconds. The oscillations of 
the suspended disk, when near its sighted position, were found to subside very rapidly, 
the energy of the motion being expended in pumping the air through the narrow aper- 
ture between the guard-plate and the suspended disk. 
The electrical arrangements were as follows : — 
One electrode of Mr. Gassiot’s great battery was connected with a key. When the 
key was pressed connexion was made to the fixed disk, and thence, through Mr. Wil- 
loughby Smith’s resistance-coils, to a point where the current was divided between the 
principal coil of the galvanometer and a shunt, S, consisting of Mr. Jenkin’s resistance- 
coils. These partial currents reunited at a point where they were put in connexion with 
the other electrode of the battery, with the case of the instrument, and with the earth. 
Another battery was employed to send a current through the coils. One electrode 
of this battery was connected with a second contact piece of the key, so that, when the 
key was pressed, the current went first through the secondary coil 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 torsion-balance 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 power- 
ful, 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 pro- 
duced 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 adjustment 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 galva- 
nometer-needle balanced each other. 
When a satisfactory case of equilibrium had been observed simultaneously at the gal- 
vanometer 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 
