SPECIFIC INDUCTIVE CAPACITY OF DIELECTRICS. 
575 
denser B, the potential of the system after the distribution of the charge is less than 
half the original potential of A and p. The combined action of the two sides p and p' 
is now less than the original action of the one sidep. Negative electricity is thus set 
free, and this, acting on the needle, deflects it to the left. 
If B be the charged condenser, or if A be charged negatively, the effect in the last 
two cases will be reversed. If both the condensers be charged, one positively and the 
other negatively, the effect will be greater in degree, but precisely similar in kind. 
This is the method usually adopted, as it gives more marked indications without 
increasing the risk of failure of insulation. 
In measuring any condenser by comparing it in this way with the sliding condenser, 
the latter is, in accordance with the indications thus given by the electrometer, so 
adjusted that its capacity becomes equal to that of the condenser to be measured. 
In order to find from this the true value of the measured condenser, it is necessary to 
know the value of the sliding condenser in scale-divisions when its index is at zero on 
the scale. 
For this purpose a condenser was employed of such a form that its capacity could be very 
accurately determined in absolute measure* (see fig. 4). This consists of two metallic 
spheres of different diameters placed one within the other. The outer consists of two 
brass hemispheres ( a a, a! a'), having their inner surfaces accurately turned, and having 
projecting flanges which are firmly fastened together by three brass screws (one of which 
is shown at c ). In the top of the upper hemisphere is a small hole through which 
projects an electrode ( dd ) from the inner sphere ( bb ), which rests upon three vulcanite 
pins (one of which is shown at e), so as to present an accurately spherical surface truly 
concentric with the inner surface of the outer sphere. 
In order to determine in absolute measure the electrostatic capacity of this condenser, 
its dimensions were carefully ascertained in the following manner. The quantity of 
water contained by the outer sphere was measured, and found to be 7 394*8 grains at 
temperature 15 0, 5 Centigrade, or 479‘66 cubic centimetres, which, with the necessary 
correction for the vulcanite pins, ‘287 cubic centimetre, gave as the radius 4 - 857 centi- 
metres. The radius of the inner sphere was obtained from the content of the outer 
sphere and that of the space between the two spherical surfaces. Of the content of this 
space five determinations were taken. These are shown in the following Table. The 
values in the second column include the correction for the volume of the vulcanite pins. 
Grains at 15° - 5 C. 
Cubic centimetres. 
1465 
95-313 
1467 
95-443 
1471 
95-702 
1469 
95-573 
1468-5 
95-540 
* The absolute unit of electrostatic capacity referred to in this paper is the capacity of an insulated spherical 
conductor of one centimetre radius placed at an infinite distance from all external objects. 
