CAPACITY AND RESIDUAL CHARGE OP DIELECTRICS. 
133 
[A.dded January IStli, 1897.] 
Castor Oil. 
This oil was obtained from Messrs. Hopkin and Williams, and was tested as 
supplied. The platinum plates, fig’. 2, were submerged in this oil. Resonance 
experiments give, for frequency 2 X 10®, a capacity equal to 105 divisions on the 
large slide condenser. For long times the method was not that shown in fig. 3, but 
a bridge method, used in the earlier experiments,* in which a Ruhmkorff coil is used 
for exciting. This test gives 139 scale divisions on the same slide condenser. In 
air the plates have capacity 30 scale divisions. We see, therefore, that at frequency 
2 X 10® the specific inductive capacity would be 3’5 as against 4'63 for long times. 
The short-time contact experiments, fig. 5, give the results in Table IX., the 
temperature of the oil being 6°C, from which we see that residual charge in this oil 
is considerable. The total capacity after time of contact ’006 second is *00034 ; 
whereas, with high frequency by resonance, it is *000287 microfarad. The curve 
in fig. 9 gives the relation between conductivity and time of contact, and has been 
plotted from Table IX. 
Fig. 9. 
* See ‘ Proc. Roy. Soc.’, vol. 43, p. 156. 
