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might, and occasionally do, accrue from using keys in which all contacts are procured by pressure pure and simple.
Condenser Keys.-A familiar key much used in condenser work is here illustrated both diagrammatically and in toto. It consists (Fig. 27) of two brass springs, A and B, connected by a bridge-piece C, which carries a terminal as shown, and also serves as a support for the springs,
being mounted on a horizontal ebonite arm F, carried by a short ebonite pedestal mounted in the base. The springs A and B are provided with the usual platinum-tipped contact studs on the under side of their free extremities, which make contact when depressed by the finger-pieces with two similar fixed contact studs, D and E, which are connected to the remaining terminals of the apparatus.
Lambert's Condenser Discharge Key by the India-Rubber, Gutta
Percha, and Telegraph Works Company, Limited. The mode of using this key is as follows:- The terminal C, and, consequently, both of the springs A and B, are connected to one side of the condenser, one of the remaining terminals is connected to the battery circuit, and the other to the galvanometer. Depressing the spring on the battery side of the key for a definite period charges the condenser and the key is then released, thus leaving the latter insulated; the opposite spring is then depressed, and the condenser immediately discharges itself through the galvanometer circuit.
Webb's Discharge Key.-A key which performs a similar office to the above is illustrated below, and is known as Webb's condenser discharge key.
A (Fig. 28) is a solid brass lever of rectangular section, pivotted at a, and normally maintained in contact with the upper pin, B, by means of the spring b; when depressed by
means of the finger piece, it breaks contact with B, and makes with C below. D is a pivoted ebonite arm carrying a detent, d, engaging with the end of the lever A, which is “stepped” as shown. The three terminals are indicated, as before, by circles, and the modus operandi is extremely simple. A is connected to the condenser, B to the galvanometer, and C to the battery. Depressing A, the condenser is brought into connection with the battery for charging. The lever is automatically held in this position by engaging the detent d, with the top step ; at the end of a predetermined charging period, the lever D is drawn out slightly, allowing A to release itself, under the influence of the spring b, as far as the second step; in this position A, and consequently the condenser, is insulated. On totally withdrawing the lever D, with its accompanying detent, A is released, and makes contact with B, thus discharging the condenser through the galvanometer circuit.
Plug Switches.—The ordinary battery connections of a testing set are usually brought direct to a plug switch from which they are again led off to the instruments. These plug switches are of multifarious design, from the simple reversing switch, shown in Fig. 29, to a regular distributing board, consisting of a series of massive brass blocks, each connected to a desirable point in the battery, such as 1, 5, 10, etc., cells, and brought into connection as desired with
common 'bus bar ny means of the accompanying plug. The switch shown in Fig. 29 consists of four brass segments, 1, 2, 3, and 4, mounted on
an insulating ebonite
base piece. The battery leads are brought to two opposite ones, such as 1 and 4, and the leading wires from the remainder of the apparatus to the other two, 2 and 3. The simultaneous insertion of two plugs on one or other of the two diameters of the circle connects the battery with the instruments in one or other direction, as may be desired.
The foregoing descriptive series of instruments and apparatus will serve to complete this section. There are one or two combinations of apparatus with which the reader will be acquainted at a later stage, as it is necessary in the first instance for him to master the elementary principles of simple testing in order that he may the more readily grasp the advantages and uses of such combinations.
We will pass on, therefore, to the practical application of the foregoing details to actual testing.
PRACTICAL TESTING. (1) Continuity, or Circuit, Test. The simplest test of all, and one which forms the actual basis of the majority of subsequent tests to be dealt with in this series, is the simple continuity, or circuit test, the object of which is to ascertain the existence or non-existence of a complete circuit through which it is possible or impossible, as the case may be, for an electric current to flow.
Referring to Fig. 30, which represents a simple circuit containing a battery E, and a galvanometer G, we note that the current from the battery will, if the circuit be complete, as shown in the figure, flow through the galvanometer by way of a and b, and a deflection of the galvanometer needle will result. If, however, we destroy the continuity of the circuit by severing one of the connecting wires, as, for example, at the point b, the current will cease to flow, and the galvanometer will remain passive. This is the principle of continuity testing. The instruments and apparatus required for the test are a battery and galvanoineter of the type known as “detector,” the construction of which was indicated in Fig. 2; as regards the battery, the number of cells required depends upon the nature and resistance of the circuits to be tested, but, for ordinary electric light, telegraph, and telephone work, where the circuit resistance does not exceed two or three hundred ohms, a couple of cells will be ample for the purpose.
The method of procedure is exceedingly simple. The battery E and galvanometer G are connected up as shown in Fig. 31, a and b being flexible leads of a suitable length for the work in hand. These leads are applied simultaneously to the two extremities of the circuit under test; if a deflection be obtained on the galvanometer then the circuit is complete; if, on the other hand, no deflection be obtained, there is a solution of continuity, and the fault must be localised.
To localise a disconnection the best plan is to divide the circuit up into suitable sections; thus, if it be an ordinary branch of electric light house wiring, from the switchboard to a lamp or lamps, the wiring from switch to ceiling-rose may be regarded as one section,ceiling-rose to lamp-holder as another, and so on; in this way it is an easy matter to find out in what part of the circuit under test the disconnection exists, and to reinedy it accordingly.
In many cases what is known as an intermittent disconnection will occur, whose presence may be detected in the following manner. The leads a and b are connected to the two extremities of the section under test, and the gal: vanometer carefully watched ; if, as before, there be no deflection at all at the end of a few minutes, then, the disconnection may generally be regarded as total, but if intermittent oscillations or flickings of the needle be obtained, it signifies a disconnection in which the severed parts are periodically making contact with one another ; cases of