upon Ohm's familiar law, E equals C.R. In this method, the resistance of the voltmeter is first measured by one of the ordinary methods at its normal temperature, i.e., at that temperature which its winding will have when connected across an electromotive force equal to that which it is ultimately intended to measure. This resistance having been duly obtained and noted, attention must be given to Fig. 91, in which the same lettering is used as in the previous figure. It will be seen on reference to this figure that the voltmeter V, testing E.M.F., E., and resistances R, r, are connected in series with the bridge, whilst the standard cell Es and galvanometer G are in shunt.

The method of conducting the test in this case consists in inserting resistances in the bridge, and adjusting R and r until there is no deflection on the galvanometer G, then, by Ohm's law, the current will be equal to the E.M.F. of the standard cell Es, divided by the resistance as read on the bridge coils. The value for C thus obtained, multiplied by the previously measured voltmeter resistance, gives the true reading of the voltmeter V at the time.

A somewhat rougher method of voltmeter calibration by the aid of a metre bridge is represented in Fig. 92, where A B is the homogeneous slide wire of an ordinary metre bridge, the resistance of which, per unit of length, must be known. R is an adjustable resistance of such a type that it will not appreciably heat with the current passing through it under the maximum E.M.F. to be recorded on the voltmeter V. E is the testing battery, as

before, fitted with a multiple contact switch C, by which the number of cells in circuit, and consequently the terminal voltage, can be varied at will. Es is the standard cell, and G the shunted galvanometer arranged in derived circuit as shown, one connection being adjustable as to its point of contact with the wire A B, by means of the slider S.

The modus operandi is as follows:-A given number of cells of the battery E, approximately yielding an E.M.F. equal to one of the required readings on V, are switched in circuit, and, by means of R, the resultant deflection on G is so regulated that the slider S is on or about the centre of the scale. S is then adjusted until a balance is obtained on G, a final adjustment being made with the shunt removed; then E.M.F. recorded on E.M.F. of standard cell (resistance A B R)

V =

resistance A S.

Ammeters.-These instruments are usually calibrated commercially, like voltmeters, by comparison with a standard instrument. For the accurate calibration of the original standard two systems are available, one of which, essentially a laboratory method and requiring the employment of delicate apparatus, is by voltameter, and the second, with which we will now proceed to deal, is by standard cell and calibrated resistance.

Professor Fleming's method of ammeter calibration by this system consists in the following:-A series of resistances are constructed, of convenient dimensions, and their actual resistances and conductivities carefully measured and noted. They are fitted with an arrangement of mercury cups so that they may be connected in parallel or in series at will. A certain number of them are then arranged in multiple such that they will permit the passage of a certain current within the range of the instrument to be calibrated, under a convenient difference of potential. This E.M.F. is measured by the standard cell and slide wire or potentiometer method, and the actual current passing is then calculated by Ohr's law. In the event of any heating of the resistances they are at once connected in series, and their resistance measured. The number of resistances is then altered,

and a second reading obtained, and so on throughout the range of the ammeter scale.

A very cute and practical method of ammeter calibration was devised some years back by Mr. Evershed, and consists in first constructing, with the utmost care, a

standard instrument to indicate exactly one ampère. This is connected in series with the ammeter to be calibrated and a convenient resistance and source of current. Several other resistances exactly equal to that of the standard instrument are constructed, and, by connecting them in parallel with it, the calibrating current can be varied ampère by ampère throughout the range of the scale,

R'

All methods of ammeter calibration by the aid of a standard cell and resistance are based on Ohm's law, E C = and, to this end, the resistances employed require to be very accurately constructed and calibrated, if any degree of accuracy in the ultimate calibration is to be attained. Messrs. Elliott Bros. manufacture a series of standard resistances to carry heavy currents, of which the accompanying illustration is typical. They are so constructed that the temperature and consequent resistance error on the passage of the maximum current specified shall not exceed 0.25 per cent.

For roughly checking an ammeter, a home-made resistance of the above type may be conveniently constructed by selecting, say, 10 ohms of German silver or platinoid wire of convenient current-carrying capacity, and cutting it up into ten equal lengths, which should have a resistance of 1 ohm each. If these be then carefully soldered across two massive copper bars, a very convenient resistance of 0.1 ohm is the result. Other values may be constructed in like manner, care being taken to allow a slight margin for the actual soldering, which should be kept within that margin.

In the various calibration methods for voltmeters and ammeters enumerated above, too much stress cannot be laid upon the subject of insulation, which should be as perfect as possible in the case of all and every piece of apparatus used, especially the batteries, or serious errors will creep into the results.

The tangent galvanometer is principally employed in the system of daily tests for insulation on all their telegraph circuits and lines, as instituted by the G.P.O. The system was devised by Mr. A. Eden, and is illustrated by Fig. 93, where G represents the P.O. tangent galvanometer, with its coils arranged differentially, E the testing battery, and r, rl rl two equal resistances of 10,000 ohms introduced between the extremities of the two line terminals and the galvanometer G. The lines are looped at their distant extremities as shown.

The modus operandi is as follows:-One winding of the galvanometer G is first connected with a low resistance battery through a standard resistance of 20,000 ohms, and the battery voltage is regulated until a con