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tive electrode, whilst C is a pencil of chloride of silver into the upper extremity of which is cast a silver connecting wire which leads to the exterior of the cell. This chloride of silver pencil is usually enveloped by a sleeve of parchment paper, and constitutes the negative electrode. The electrolyte, E, is a solution of 23 grammes of pure sal-ammoniac in one litre of water. This cell has an E.M.F. of 1.03 B.A. volts.

Still another form of battery which has been largely adopted for testing purposes is the Minotto modification of the Daniell cell. It consists, Fig. 24, of a glass or highly-glazed stoneware containing jar A, at the bottom of which is placed a disc of copper, B; a copper connecting wire is rivetted to the latter, and, insulated with

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gutta-percha, is led up at the side to the exterior of the cell. On the copper disc is placed a layer of copper sulphate crystals, C, then a disc of thin cancas, d, surmounted by a layer of sand or sawdust, D, the former for stationary cells, the latter for the portable type. On this again is placed another disc of thin canvas, d, and finally the thick zinc electrode, E, on which is cast a brass terminal

cap, The electrolyte consists of a solution of zinc sulphate in water, which is poured in so as to cover the zinc plate,

F.

E. It is better, when primarily setting up the cell, to moisten the sand or sawdust in the first instance with the electrolyte, squeezing it almost dry again before placing in the cell, as this proceeding facilitates the permeation of the liquid, and consequent rapid working up of the cell. Its E.M.F. is approximately that of the Daniell cell proper, viz., 1.079 B.A. volts.

Secondary batteries or accumulators have also been applied to testing purposes, and, from their comparatively constant discharge rate, would impress one as being eminently suitable, but they require constant attention and perfect insulation, the former being a matter which involves time and labour, and the latter a condition not easily obtainable in any enclosure containing secondary cells, owing to the acid fumes, etc., emitted thereby, so that they have not been altogether a success. Of course, there are some cases where, in dealing with extremely low resistances, a comparatively heavy current of short duration is needed for a small period; where this is 80, accumulators are a necessity, but a stand-by of from one to five cells will generally be found ample for anything of the kind.

Thus far the writer has only dealt with three distinct types of primary battery for testing purposes; there are, of course, a hundred and one other types, all capable of yielding a current more or less constant; but this does not profess to be a work on the ubiquitous primary cell, and the types enumerated above are those more generally adopted for testing purposes, and will therefore serve the purpose of the writer by comprising the battery section of instruments and apparatus.

Keys, Switches, dc.-Of keys, switches, and contact devices there are many types. It will serve our purpose fully if we deal with the principal ones here, viz., those which will be essential to the subsequent tests to be described in these pages.

The Simple Circuit Key.This useful piece of apparatus comprises a device for closing or completing any circuit at will, and is usually constructed as shown in Fig. 25, where A consists of a flat brass spring, mounted at one extremity on an ebonite block or insulating pillar B, and free at the opposite extremity as regards its vertical movements within certain limits, which enable it to "make" and " break” contact at will with the stud C, which is platinum tipped, as well as the corresponding point on the spring, in order to reduce the electrical resistance due to oxidation at the point of contact. D is a finger piece used to depress the spring, and it is generally provided with a locking device E, engaging with

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the stud F, which enables the operator to leave the key closed for any required period. H is one of the terminals, the remaining one being in a line with it, and therefore not seen in the figure. They are, of course, connected with the spring A and the stud C respectively.

The Short-Circuit Key.—This is an extremely useful key, and is essential to the safe working of all sensitive galvanometers. It is connected across the terminals of the instrument, and its office is to form an alternative path of negligible resistance for the current, or, in other words, to short-circuit it and thus put it out of action.

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Galvanometer Short Circuit Key with Locking Piece, by the India

Rubber, Gutta-Percha, and Telegraph Works Co., Ltà.

It is an exceedingly simple key, consisting of a straight brass spring, mounted on an insulated base and provided with a terminal. Normally, this spring rests in contact with the platinum-tipped extremity of a screw mounted in a bridge piece, which is also connected to the remaining terminal. At its free extremity the spring is provided with an ebonité finger-piece, by means of which it can be depressed out of contact with the second terminal on to an insulating ebonite stud. To keys of this type illustrated above a revolving ebonite catch, engaging with a fixed stud on the base, is added, in order that the key may be kept depressed automatically for any desired length of time without the attention of the operator.

The Reversing Key.This useful piece of apparatus, illustrated below, is employed, usually in the battery circuit, for connecting either pole of the latter to the circuit at will, or,

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Ordinary Pattern Pillar Reversing Key on Ebonite Base

by Nalder Brothers. when necessary, for totally disconnecting it. The key is represented in diagram by Fig. 26, A and B being the two brass contact springs, which are mounted on insulating ebonite pillars fixed in an ebonite base piece, as shown in the completed illustration above. The springs pass under, and normally make contact with, platinum-tipped screws in the bridge-piece C, whilst at their free extremities they are provided with platinum contact studs, arranged opposite to similar studs on the bridge-piece D below. All these several parts are, like the springs A and B, mounted on insulating pillars, so that the key is highly insulated, a necessary precaution in most tests. Four terminals are provided, as shown by the small circles, the battery being connected to the two marked (+) and (-) and the remainder of the circuit to the other pair.

The action of the key will be easily understood ; if one of the springs, A, for example, be depressed, it is brought into connection with the (-) negative pole of the battery, whilst the remaining 'spring, B, is in contact with the (+) positive pole and vice versa. The direction in which the battery current is flowing through the circuit may thus be reversed at will, or checked altogether by simultaneously releasing both springs.

This key is a facsimile of the double “tapper” key used in single-needle telegraphy. In some types the contact

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Fig. 26. bars C and D are divided at the centre, as indicated by the interrupted line a b, and provided with terminals at either end; when this is the case, the two halves can be disconnected when desired, and employed as separate keys.

Like the preceding one, this form of key is also provided with a device for automatically holding the springs down when depressed. It consists of an ebonite cam, engaging with the top of the spring, and mounted on a horizontal spindle, which is actuated by a suitable handle, and revolves in a trunnion, which, like all other parts of the key, is mounted on an insulating pillar; one of these cams is fitted to either spring.

In some cases, the foregoing key takes the form of the reversing switch shown below, which was designed by Mr. J. Rymer Jones. In principle and resultant action, it is precisely similar to the reversing key described above, and possesses the attendant advantage that all contacts made through it are of a sliding or rubbing nature, thereby eliminating all danger from bad contacts, etc., which

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