THE SPECIFIC INDUCTIVE CAPACITY OF A DIELECTRIC. 
3 
The condensers needed several heatings before settling down to a steady capacity in 
the cold state. The temperatures in these preliminary heatings were always above 
the highest used in the actual observations. 
The capacities were measured by the method used by Professor J. J. Thomson in 
his determination of the ratio of the electrostatic and electromagnetic units 
o 
(‘Phil. Trans./ 1883). It is thus described in his paper ;— 
“ In a Wheatstone’s bridge, A B C D, with the galvanometer at G, and the battery 
between A and B, the circuit B D is not closed, but the points B and D are connected 
with the two poles, E, and S, of a commutator, between which a travelling piece, P, 
moves backwards and forwards; P is connected with one plate of a condenser, the 
other plate of which is connected with D. Thus when P is in contact with S, 
the condenser will be charged, and until it is fully charged, electricity will flow 
into it from the battery; this will produce a momentary current through the various 
arms of the bridge. Wlien the moving piece P is in contact with R, the two 
plates of the condenser are connected, and the condenser will discharge itself through 
L) R, and as the resistance of D R is inflnitesimal in comparison with the resistance of 
any other circuit, the discharge of the condenser will not send an appreciable amount 
of electricity through the galvanometer. Thus, if we make tlie moving piece P 
oscillate quickly from R to S, there will, owing to the flow of electricity to the 
condenser, be a succession of momentary currents through the galvanometer. The 
resistances are so adjusted that the deflection of the galvanometer produced by these 
momentary currents is balanced by the deflection due to the steady current through 
the galvanometer, and the resultant deflection is zero. When this is the case there 
is a relation between the capacity of the condenser, the number of times the condenser 
is charged and discharged per second, and the resistances in the various arms of the 
bridge.” 
This relation, which is worked out in the paper, is expressed by taking a, h, c, c/, g 
to represent the resistances of A C, A B, A D, B C, D C respectively. The resistances 
