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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 ci 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 = 0
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 Ci, 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 Ci 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 di is noted,
d - dl then C1
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 Ci, 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 Cfrom ci, 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,
the short circuit key is opened and the galvanometer G thereby introduced into the circuit. A second deflection
С dl is thus obtained, then Cl =
d 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
S resulting throw dl is noted, then C
d 2.303 R log
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
galvanometer G, due to the full potential of the charging battery. K is then opened after a suitable charging period has elapsed, and the deflection on the galvano
meter gradually falls owing to the discharging of C through the resistance R. The reduced deflection di at the end of S seconds from the commencement of the
S discharge is noted, then C
d 2.303 R log
In both the above tests C is in microfarads, Sin seconds, and R in megohms.
Having thus far dealt with the elementary principles for the practical determination of the four fundamental electrical quantities, viz., Resistance, Electromotive Force, Current, and Capacity, we will row proceed to discuss one or two modifications and combinations of the foregoing methods, together with the apparatus necessary for their conduct, which are applicable to certain branches of electrical work.
Cable testing, in its many and varied forms, probably calls for the largest combination of electrical tests to discover its various qualities before being put on the market, and after being fixed or laid in position, in order to ascertain if it is suitable for the work it is intended to
perform in the transmission of electrical energy from one point to another.
Electric cables and wires are tested for insulation resistance, electrostatic capacity, obmic resistance, and, in the case of high tension work, for resistance to disruptive discharge, both at the factory during manufacture and by the purchaser when fixed in position, before absolutely putting them to work, and several very useful combinations of apparatus have been designed, with a view to including two or more of these various tests, and the switches, shunts, resistances, etc., necessary to their manipulation upon one universal base for the sake of portability and general convenience. Of these special combinations we will select two, and deal with them in turn as a pattern upon which are moulded the remainder of their species.
The Silvertown Portable Testing set, a most popular