CAPACITY AND RESIDUAL CHARGE OF DIELECTRICS. 
119 
iDlaciug the bridge between 1, 4 and 3, 4, and adjusting E, the potentials are brought 
into the same phase, that is, the potential across the electrometer is zero in each case 
for the same position of the contact-maker. Mercury cups 2, 4 are now connected, 
the contact-maker B is adjusted to the point of maximum potential, and E^, Ej 
adjusted until balance is obtained. We now know that K/F = Ej/Eg. 
A ‘3 is the ordinary key supplied with the electrometer, which reverses the charge on 
the quadrants or short circuits them. The range of frequency varies from 1.00 to 7 
or 8 complete periods per second. 
( b .) High Frequency. 
For high fre([uencies a method of resonance is used,"^^ and the apparatus shown in 
fig. 4. The primary coil consists of 1, 9, or 160 turns of copper wire 4 feet in diameter, 
having a condenser in its circuit and two adjustable sparking knobs «, h. The 
secondary is placed with its plane parallel to that of the primary, and usually at a 
distance of 4 or 5 feet from it; adjustable spark knobs c, d are provided in its circuit, 
which consists of 1, 9, or 160 turns of copper wire of the same diameter as the primary. 
The diameter of the wires for the 1, 9, and 160 turns are respectively 5‘3, 2'65, and 
1 '25 millims. A Euhmkorff coil excites the primary. Between the spark knobs c, d. 
are placed the capacity to be found F, and a large slide condenser K. The method 
is one of substitution, that is to say, maximum resonance is obtained with both 
condensers attached by variation of K ; F is removed and maximum resonance a,gain 
obtained by increase of K, In order to bring K on the scale for the two maxima, it 
is necessary to adjust Kj, the condenser in the primary. This condenser consists of a 
* Tins method, we find, has been used by Thwino, ‘ Physical Society’s Abstracts,’ vol. 1, p. 7b. 
