It is then released, and K1 depressed in its turn; this action serves to discharge C through the galvanometer, the magnitude of the initial swing of the needle of which should be noted; we will call it d. The standard condenser C is then disconnected and replaced by the condenser under test, which in the case of our length of cable, for example, would consist of one end (the other being free), and the earth surrounding it, such as the armouring or lead sheathing, if it possessed any, or the water in a tank if the cable be submerged therein. The operations with K1 and 2 are then repeated, and a second deflection or swing dl is obtained.

Then the capacity under test: C :: d1: d

C dl

or, the required capacity = d

If a shunt be used in either of the above operations, the necessary multiplication of the observed deflections G+S, by the now well-known formula must, of course, S be effected.

Gott's method is illustrated in Fig. 64, where C and C1 represent the standard and the condenser under test respectively, E the battery, K a Webb's discharge key previously described, K1 an ordinary circuit key, G the galvanometer, and r, rl two resistances giving a fairly large

range of proportional adjustment, with attendant accu-
racy of comparison. r and rl are first adjusted as nearly
as possible in the proportion of C to C1; K is then de-
pressed, and held in that position by means of its detent,
thus charging both C and C1 simultaneously in series,
from the battery E. After a definite period as before, K1
is closed, and, if a deflection be obtained on the galvano-
meter G, both keys are released, and the respective con-
densers short-circuited, or put to earth as the case may
be, in order to dissipate their respective charges.
rl are then readjusted, and the operation repeated until
no deflection results upon the galvanometer when Kl is
depressed.

r and

The proportional resistances r and r1 may conveniently be arranged in the form of a slide resistance of some 10,000 ohms, the galvanometer being connected to the slider.

Thomson's method of capacity measurement is very similar to the foregoing, the connections being represented in Fig. 65, where C and C1 represent the standard and condenser under test respectively, E the battery, G the galvanometer, and 1, 2, 3, 4, and 5 simple circuit

keys, all of which may be dispensed with by employing a combination key designed by Mr. Lambert, which combines all the necessary movements upon one universal base.

The method of conducting the test is as follows: Key 1 is first closed, thus connecting the battery E through the resistances r and rl to earth. Keys 2 and 3 are then closed simultaneously (by a single movement in Lambert's key) for a definite period, in order to charge the two condensers C and Cl; the two Keys 2 and 3 are then opened, and Key 4 is closed also for a definite period in order to allow the respective charges in condensers C and C1 to mix. Finally Key 5 is closed, and, if a deflection ensues, the ratio of r to rl is varied, and the same operations repeated until no deflection results upon the galvanometer when 5 is closed, then C1 = C/1

What is known as the Divided Charge method for the measurement of electrostatic capacity is represented in diagram by Fig. 66, where E represents the charging

battery as before, G the galvanometer, C the standard condenser, and K a Webb's discharge key. C is first charged by depressing K on to the lower contact for a definite period. K is then released, and makes contact with the upper stud, thus discharging C through the galvanometer G. The resulting deflection or throw d is

noted, and C is again charged by depressing K on to its lower contact for a similar period; at the end of the charging time the lever of K is set by means of its detent in the centre or insulated position shown in the figure, and C1, the condenser under test, is substituted for the battery E, care being taken not to touch the lever of K (the safer course would be to arrange C1 in a parallel circuit so that it can be substituted for E by means of a plug or other well-insulated switch). K is then again depressed, thus connecting the two condensers and allowing a consequent division between them of the initial charge.

This connection is left on for a definite period, as usual, and the standard condenser C is then again discharged by releasing K, so that it makes contact with the upper stop, and the second deflection dl is noted, d dl then C1 C

di

The familiar name of Siemens is associated with three tests for the determination of electrostatic capacity, the first of which, known as Siemens' Diminished Charge method, is somewhat similar to the foregoing divided charge method, and possesses the attendant advantage of being applicable to the measurement of a large capacity by comparison with a standard condenser of small capacity.

The connections for the test are the same as indicated in Fig. 66, with the exception of an additional short circuit key, which must be introduced across the terminals of the galvanometer G. The mode of procedure is as follows:-Referring to Fig. 66, K is first depressed, thus charging the standard condenser C for the usual period; it is then released, and the discharge deflection d noted. K being then set at "insulate," as before, the parallel circuit, including C1, is brought into connection with E and charged for a like period. E is then cut out of circuit, and the standard condenser charged from C1 for the same period, and subsequently discharged, the galvanometer short circuit key being closed meanwhile. This charging of C from C1, and subsequent discharging through the short circuit key, is repeated for a given number of times, x, the charge remaining after each discharge, being less and less, until, at the x th. discharge,

F

the short circuit key is opened and the galvanometer G thereby introduced into the circuit. A second deflection

dl is thus obtained, then C1 = √

C

For great accuracy in this test it is necessary that all apparatus should be well insulated to minimise leakage, and that the charging and discharging be effected with as small a time interval as possible.

Siemens' Loss of Charge Discharge method for the determination of electrostatic capacity consists in comparing the total discharge from the condenser under test with that discharge which takes place after the initial charge has been dissipating itself for a stated period through a known resistance of high value connected across its terminals.

As before, Fig. 66, with the single addition of a high resistance, usually some hundreds of megohms, across the terminals of C, which, in this case, denotes the condenser under test, will serve to explain the system. K is first depressed to charge C for the usual period, at the end of which it is immediately discharged by releasing the lever on to the upper contact, giving a discharge deflection or throw d. It is then depressed to charge again for the same length of time, at the end of which the lever of K is freed to the "insulate" position for a given period, say 30 seconds, during which time C will discharge itself in part through the high resistance which we will call R. At the end of the stated interval or number of seconds, S, the lever of K, is again released to discharge, and the

resulting throw dl is noted, then C

=

S

2.303 R log

d dl

Siemens' Loss of Charge Deflection method is somewhat similar to the foregoing, and is represented in Fig. 67, where C is the condenser under test, connected at will for charging purposes to the well-insulated battery E, by means of the key K. G is a galvanometer also connected with the terminals of C through a high resistance R, which, as before, runs into megohms. The test is conducted as follows:-K is first closed, thus charging the condenser C, and producing a deflection d on the