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 E 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 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 current C = El 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 strument may be synonymous with that of the instrument itself. Resistance of a Partial Earth Fault.-A rough method for the determiration of fault resistance by the aid of a high resistance voltmeter is due to Swinburne, and consists in first measuring the testing voltage by means of the said voltmeter connected across its terminals, and then obtaining a second, lesser reading from the voltmeter in series with the fault, then Resistance of Leak + Resistance of Voltmeter Ammeter and Voltmeter Calibration. This series would not be complete without a more or less detailed reference to the subject of calibration, or, in other words, the comparison of a commercial instrument with a standard, for the purpose of preparing a scale for, or correct system of, reading the indications of the former. The usual plan adopted in factories where commercial instruments are constructed consists in first standardising one individual instrument by one of the methods to be described, and then adopting it as a standard of comparison with the remainder, checking it from time to time by the original method, as circumstances and previous experience dictate. I will now proceed to deal with one or two methods for calibration of the commercial standards, involving the use of simple apparatus, and, as an opening, we will consider the all-important section included under the heading of Voltmeters. For voltmeter calibration we require a source of E.M.F. capable of supplying a voltage at least equal to the maximum reading on the scale of the instrument under calibration, and this is best supplied by a battery of accumulators. If an ordinary current-yielding set of accumulators be not obtainable, the requirement may be met by one of the many patterns of testing battery on the market, most of which consist of a pair of lead strips, pasted or "formed" in the usual manner, and immersed in an electrolyte contained in a small test tube. If such a set be adopted for the purpose in hand, it must be borne in mind that no appreciable current can be taken from them without materially lowering the ter- A simple method of voltmeter calibration, described ton and Co. in the earlier days of instrument manufacture, is depicted in Fig. 90, where a, b, and c represent, as usual, the proportional and adjustable arms respectively of a Post Office Wheatstone bridge. V is the voltmeter to be calibrated, E the source of E.M.F. previously alluded to, Es a standard cell (usually Clark's). K2 is an auxiliary circuit key, R an adjustable resistance arranged in convenient form, so as to give a wide range of adjustment, r being an auxiliary rheostat or slide resistance for final balancing in reading to single degrees of the voltmeter scale. rl and r2 are special resistances of 54.6 and 145.4 B.A. ohms respectively, introduced in order to render the arrangement direct reading in B.A. volts, their total resistance being 200 B.A. ohms. In the event of standards other than B.A. being adopted, the same sum of resistances must be adhered to. G is a galvanometer. As to the actual manner of conducting the operation, I cannot do better than quote Mr. Swinburne's own remarks on the subject: "R, r, in Fig. 90, are adjustable resistances and rheostat coupled up as shown. This resistance box is supposed to have coils for the thousands, so that when all the plugs are out 11,110 ohms are in circuit; rl and r2 are two specially made up resistances of 54.6 and 145.4 ohms respectively. The Clark standard cell is in series with a galvanometer G. The standard cell circuit is from between r1 and r2 to the keys, so as to shunt the resistance of 145.4 ohms. If the plugs be withdrawn between the extremities of arm a, there is a resistance of 1,110 ohms in the standard cell circuit. The object of the resistance rl is to make up 200 ohms with r2, so that the apparatus may be direct reading. Suppose the voltmeter is to be calibrated up to 100 volts. To get 50 volts on it, plugs are drawn in the resistance box to give 4,800 ohms, which, with rl and r2, make up 5,000 ohms. The resistances R are then adjusted till approximately 50 volts are on the voltmeter. The key K1 is then pressed for a moment. The galvanometer will be deflected, showing that there is too much or too little electromotive force. The switches R can regulate within 1 per cent., so that a pair of positions will be found, one on each side of the desired resistance. The rheostat r is then adjusted till there is no deflection on pressing K1. The key K is then pressed, and a final adjustment made. The key Kl and its resistances are put to prevent a large current passing through the cell in either direction, and the key K must only be used for final adjustment. The 50-volt reading having been taken, 5,100 ohms are drawn, and the 51-volt reading is taken, and so on." An alternative method, which, however, possesses the attendant disadvantages that it is not direct reading, and that it cannot be adopted except in the case of high resistance voltmeters, the maximum current through the coils of which will not be sufficient to appreciably heat the bridge coils, is represented in Fig. 91, and is based |