.s/A' WILL/AM SI KM ENS, F.K.S. i;i 



coil, X, and the other through the unknown resistance, X'. This 

 unknown resistance, X', is represented to be a pyrometer-coil de- 

 scribed in the Second Part of this Paper. 



By turning the commutator seen at Fig. 9 either in a right or 

 left hand direction from its central or neutral position (in which 

 position the contact springs on either side rest on ebonite), 

 the current from the battery flows through the two circuits, 

 causing decomposition in the voltameters ; and the gases gene- 

 ral i-d upon the electrodes accumulate in the upper portions of 

 the graduated tubes. By turning the commutator half round 

 every few seconds the current from the battery is reversed, 

 which prevents polarization of the electrodes, as already stated. 

 When through the position of the commutator the current flows 

 from the copper, it passes first through the connected electrodes 

 to the voltameters, where it divides, one portion passing through 

 the constant resistance, X, through the leading wire, X, to the 

 pyrometer, returning by the leading wire, C, to the battery, the 

 other passing through X', through the leading wire, X', through 

 the platinum coil, returning by the leading wire, C, to the battery. 

 Wlu'ii the current flows from the zinc it passes first through the 

 leading wire, C, the current dividing at the pyrometer, one portion 

 returning by the leading wire, X, through the constant resistance, 

 X, tli rough one voltameter tube to the battery, and the other 

 through the platinum coil, X', through the leading wire, X', to the 

 other voltameter tube, and thence to the battery. The value of 

 the third leading wire, C, in eliminating the disturbing efl'ect 

 which long and short leading wires with varying temperature 

 would certainly have upon the correct indications of the instru- 

 ment is at once evident. 



The relative volumes, v and v', of the gases accumulated in an 

 arbitrary space of time within each tube must be inversely pro- 

 portional to the resistances, R and R', of the branch circuits, 



T? p 1 



because v : v' = - - t: . t, and, therefore, v : v' = R' : R. 



The resistances, R and R', are composed, the one of the resist- 

 ance, C, plus the resistance of the voltameter, A, and the other of 

 the unknown resistance, X, plus the resistance of the voltameter, B. 

 But the instrument has been so adjusted that the resistances of the 

 two voltameters are alike, being made as small as possible, or 



