than that resulting from the latter; if it be so, the joint is defective.

The Discharge Method is primarily the opposite of the foregoing, and consists in fully charging the condenser, which is then allowed to discharge itself for a stated period through the joint, its subsequent discharge deflection at the end of that period being duly ascertained and compared with the instantaneous discharge; if the joint be good, there should be very little difference between the two deflections.

Raymond-Barker's Accumulation Null Method of Joint Testing. Referring to the two joint tests described a few paragraphs back, it will be seen that they consist in comparing the leakage of current through the dielectric of the joint, either into or out of a condenser, and, as this leakage in the case of a perfect joint must of neces sity be extremely small, it follows that any extraneous leakage, however slight, such as that caused by the imperfect insulation of the trough T, Fig. 85, will exercise an appreciable effect upon the test, especially under varying atmospheric conditions, i.e., when the reading from the joint proper may be taken under different conditions to that from the comparative sample of perfect

core.

It follows, therefore, that a test which allows of simultaneous observations on both joint and core, is much less liable to error than one in which the observations are made separately, and the following method provides a means of attaining the required result. It is represented in Fig. 86, where T is a well-insulated ebonite trough divided into two portions, also well insulated from one another by a central partition. p, pl are contact plates immersed in water in the troughs which contain the joint under test, and the comparative length of perfect core. respectively and C1 are two equal condensers, E a battery of high E.M.F., and G a reflecting galvanometer. Ki, K2, and K3, shown respectively as two Webb's discharge keys, and a simple circuit key, are combined in Price's mixing key, whilst K4 is a second simple circuit key. The modus operandi is as follows:-K1 and K2 are depressed simultaneously (one movement of the combination key effects this), thus charging C and C1 with electricities of opposite sign through the joint and core

respectively. K1 and K2 are then released, and allow the charges to mix with a tendency to neutralisation. K3 and K4 are then closed, and any preponderance of one charge over the other produces a deflection on G, which will be to one side of the scale zero or the other, according to which possesses the higher insulation, the joint or the core. The direction of indication may readily be determined by experiment with two core samples of known inequality of insulation.

If, in the aforementioned joint tests, the core or joint form part of a circuit of appreciable magnitude, the short circuit plugs of C and C1 must, in the first instance, be inserted, and withdrawn after depressing K1 and K2.

Having thus far dealt with the ordinary routine of testing, we will now proceed to discuss certain matters which, although beyond the pale of testing pure and simple, are nevertheless connected with it, and in many cases have an important bearing thereon. The major portion of the following matter has been culled from recent publications, periodical and otherwise, and, this being the case, it will tend by its presence to bring the foregoing information up to date. We will deal with them severally under the convenient heading of

MISCELLANEOUS.

Price's Guard Wire.-The merits of this device were

recently discussed in a paper read before the Institution of Electrical Engineers, and it merits special mention in that it is a successful application for the elimination

of that bugbear surface leakage, in insulation testing, etc. The principle, as applied to the extremities of a submerged cable under test for insulation, is represented in Fig. 87, where A and B represent the two extremities of the cable, duly prepared and tapered in the manner described under the heading of Insulation Resistance Measurement, G the galvanometer, and E the testing battery. At the points a and b a fine copper wire is wound tightly for some three or four turns around the tapered extremity of the insulation, over which the surface leakage tends to take place, and connected, as shown, between the galvanometer and battery. However great the leakage tendency before this precaution was taken, it will be found to have entirely disappeared when the connections are arranged as in the figure, and only the true deflection due to the cable will be obtained.

The same principle may be applied to prevent surface leakage on the galvanometer by connecting the three levelling screws or case of the latter to the insulated battery terminal, care being taken to ensure the thorough insulation in all other respects of the galvanometer itself, as otherwise a short circuit will exist across the battery terminals.

Output of a Battery when Working.-This ingenious method was devised by Mr. I. Probert, and is specially applicable to the measurement of the discharge current from a small battery or accumulator such as those employed for the lighting of miniature incandescent lamps, etc., where the resistance of an ammeter, however low, if placed directly in circuit, would cause an appreciable reduction in the amount of current passing. The method is illustrated in Fig. 88, where E is the battery under

E

A

R

FIG. 88.

test, which, in the position shown, of the two-way switch S normally supplies current to the lamp L. V is a voltmeter of high resistance placed across the terminals of the latter. El is an auxiliary battery, A an ammeter, and R a liquid resistance, capable of minute adjustment. The method of conducting the test is as follows:-The reading on the voltmeter V is noted under the normal conditions depicted above, and S is then switched over such that the auxiliary battery El, resistance R, and ammeter A, are introduced into the circuit.

R is then adjusted until the original reading on V is reproduced; the ammeter A will then record the normal current taken by the lamp.

A Method of Measuring the Resistance of an Electric Lamp whilst in a State of Incandescence.—The following method, detailed by Kempe, is a very handy one for dealing with the required resistance of a lamp, or series of lamps, without interrupting the supply cur

rent. It is represented diagrammatically in Fig. 89, where L represents the lamp under test, which is lit by a cur rent passing from A to B as indicated by the arrow heads. R is a resistance of suitable value connected in series with it. The E.M.F. between the extremities of R

A

wwwwww

B

FIG. 89.

R is measured; we will call it E; likewise the E.M.F. at the terminals of the lamp, which we will call El. Then

El

the required resistance x of the lamp L: x = R E

Having thus ascertained the resistance of the lamp, the current consumed by it can easily be determined by a simple application of Ohm's law, for the required curEl

rent C =

Ꭱ

Test for Differentiality.-To ascertain whether a galvanometer be truly differential, its coils should be connected up in series in such a manner that the deflective effects of the currents in the two windings oppose one another, and a current passed through them. If there be any deflection, however slight, it indicates that the instrument is not truly differential, and that the deflective effect of one coil or set of coils is slightly in excess of that due to the other. If, on the other hand, no deflection be obtained with the coils or windings thus connected, they may be again arranged, in parallel, but still opposing one another, and the experiment repeated; if no deflection result, the resistances of the opposing windings are equal to one another.

To correct an error discovered by the above test additional resistance must be connected in series with one of the windings, and such resistance should preferably be of the same material and gauge as the actual winding itself, and located on the same base with the galvanometer, in order that its temperature coefficient and deflective effect (if any) on the moving system of the in