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 the shunted galvanometer arranged in de rived circuit as shown, one connection being adjustable as to its point of contact with the wire A B, by means of the ider 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) resistance A S. V = 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 Ohm'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, 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 re R' quire 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 Grepresents the P.O. tangent galvanometer, with its coils arranged differentially, E the testing battery, and r, 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 stant deflection of approximately 110 tangent divisions is obtained on its scale. The constant having been thus obtained and noted, the galvanometer is next connected as shown in Fig. 93, such that the current from the bat man LINE min LINE Fig. 93. tery E will tend, by flowing through both windings, to produce a deflection in opposite directions of the galvanometer needle. It is obvious, then, that if the insulation resistance of the line be infinite, and there be no leakage, the currents through r and rl, and consequently through the two sides of the galvanometer winding, will be equal and opposite, and, as a natural consequence, no deflection will result; if, on the other hand, there be a leakage, either local or general, at any point on either side of the lines, the current through r will be greater than that through rl, and a deflection will result. By means of a series of specially prepared tables, this deAection can be rendered in terms of the insulation resistance of the line, in “megohms per mile.” The conductor resistance of the loop naturally enters into these calculations, and is duly provided for in the tables alluded to. In the event of a single line requiring a test where no looping can be effected, the distant end is put to earth through a combined resistance of 10,000 ohms, and the winding of the galvanometer normally connected to rl, and the deflection due to the line, as calculated for perfect insulation, is deducted from the observed deflection, then the remainder, multiplied by 2, will give the required deflection from which the insulation resistance can be deduced, as before, by reference to the tables. INDE X. PAGE 50 6 13 Ballistic Galvanometer 123 54 57 58 59 38 41 43 45 46 49 71 71 73 145 38 31 33 33 35 PAGE Fault 153 38 103 86 town Portable, Method of 135 53 103 105 105 92 83 67 105 103 50 of -By Siemens' Dynamometer 116 120 122 ... J20 Daily Insulation Tests of Telegraph 185 10 15 11 17 71 69 Determination of Current Strength Cables, Ends of, Preparation of, 82 80 183 178 ment of 126 127 129 130 130 97 51 Method for Determination of 121 Determination of Current 119 Measurement of Electrostatic 125 177 63 64 173 ment of Electrostatic Capacity 128 |