4 
MR. W. CASSIE ON THE EFFECT OF TEMPERATURE ON 
of D R and S B are so small as to be negligible. Then if the condenser has a capacity 
C, and is charged and discharged n times per second 
a \ 1 
nO = 
{a + c -\-g) (a + 5 + cl) 
cd ^ 1 “h 
ab 
c{a + b + d) 
1 + 
ag 
d(a + c + g) 
The resistances used were about 
a = 10, 
b=8, 
c between 3000 and 6000, 
d „ 6000 „ 2000, 
g = 4060 ohms. 
With these values 
is correct to within ’2 per cent., and is the formula used here. 
The commutator P R S (fig. 2) was that used by Professor Thomson and is thus 
described in his paper ;— 
Fig. 2. 
‘‘ The current from two Gpove’s cells passes first through a tuning-fork interruptor, 
and then through the coils L M of an electromagnet. P N is a strip of brass with a 
piece of iron attached to it. When there is no current passing through the electro¬ 
magnet, the elasticity of the rod P N makes it press against a screw T, which is 
electi’ically connected with a binding screw R : when the current passes through the 
electromagnet, tlie magnet attracts the iron attached to the rod P N and brings it 
into contact with the stop G, which is electrically connected with the binding screw S. 
The letters P, R, S indicate the same points in this figure as in fig. 1. All the places 
where contact is made by the vibrating piece P N are covered with platinum, and the 
