[MILLER & KENRICK] ELECTRICAL MEASURING APPARATUS 103 
When the mechanism comes to rest, the voltage between [8] and the 
seraper will evidently be 1/100 of that between the terminals X of the 
instrument. But as every two contact wires on the wheel (one division 
on the dial) correspond to 1/100 volt, the dial reading gives directly the 
potential difference between the terminals in volts. 
THe CoMMUTATOR. 
In order to avoid polarization when measuring the resistance of 
electrolytes, the direction of the current in the cell is alternated by means 
of the commutator (of figs. [V. and 
V.; see also fig. VIII.). The brushes 
bearing on the inner and outer rings 
connect with mercury cups 21 and 22 
respectively, while the electrodes of 
the resistance cell communicate with 
a pair of brushes rubbing on the cen- ; 
tral (broken) ring, one behind the | 
other at an interval of one section. 
As the commutator revolves, the 
direct current to and from the mercury 
cups is interrupted, while in the circuit 
containing the electrolyte it is trans- 
formed into a series of pulsations in Bie. VIII. 
alternate directions. In order to prevent these interruptions from affect- 
ing the galvanometer, connection with the latter is broken at the right 
moment by small pieces of vulcanite, let into the margin of the outer 
ring. 
All five brushes are mounted on a hinged frame (fig. IX.) ; when 
thrown back, so as not to touch the commutator rings, they make perma- 
nent connections between [21] and [23]; [22], [24], and [26]. 


ATTACHMENT FOR MEASURING POLARIZATION. 
In order to measure directly the least electromotive force that will 
cause a current between, say, platinum wires in a silver solution, the sign 
block “1/100 Volts ” is inserted, the terminals of the instrument joined 
to the platinum wires, and the cord represented by dotted lines in fig. V. 
connected to the solenoid L. 
The contact 7’ completes the circuit containing the solenoid A; the 
wheel moves in the direction of the arrow in fig. V.; and the electro- 
motive force between the platinum wires increases until a current of 
1/100,000 ampére flows through the solution and the galvanometer coil. 
sec. III, 1900: 7. 
