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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

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STANDARD RESISTANCE FOR HEAVY CURRENTS, BY ELLIOTT BROS.

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
Accumulation Joint Test, Clark's 171
Ampère

50
Astatic Galvanometer

6
Ayrton-Mather D'Arsonval Galva-
nometer

13

Ballistic Galvanometer

123
Batteries, Leclanché...

54
Warren De La Rue's

57
Minotto's

58
Secondary

59
Clark's Standard

38
Modifications of

41
Conditions Essential to Satisfac-
tory Standard

43
Raleigh “H” Type of Standard 43
Dr. Muirhead's Type of Standard 43
Prof. Carhart's Type of Standard 44
Callendar & Barnes' “Inverted
Standard

45
Wachsmuth & Jaegar's Standard 46
Dr. Fleming's Standard

46
Grotian's Standard

49
Resistance of, by Wheatstone
Bridge Method

71
Resistance of, by Reduced Deflec-
tion Method

71
Resistance of, by Mance's Method 72
Resistance of, by Thomson 's
Method

73
Blavier's Method of Fault Locali-
sation

145
Board of Trade Specification for
Clark's Cell

38
Bridge, Wheatstone, Principle of 30
Construction of

31
P.O. Form of

33
Dial Form of

33
Dr. Fleming's Form of

35

PAGE
Clarz's Method of

Fault
Localisation

153
Standard Cell

38
Coefficients, Temperature, Tables
of, for Copper

103
Coefficients, Temperature, Dielec-
tric

86
Combination Testing Set, Silver.

town Portable, Method of
Using

135
Completo Break, Localisation of 144
Condensers

53
Conductivity, Measurement of, by
Weight

103
Conductivity, Measurement of, by
Diameter

105
Conductivity Apparatus, Nalder
Brothers'

105
Connecting Leads

92
Constant, Insulation...

83
Continuity or Circuit Test

67
Copper Conductivity Apparatus,
Nalder Brothers'

105
Copper, Temperature Coeficient's
for...

103
Cowan's Regulating Transformer 90
Current, Standards of

50
Current Strength, Determination

of -By Siemens' Dynamometer 116
By Direct Deflection Method 119
Ву Difference of Potential
Method

120
By Kempe's Bridge

122

...

J20

Daily Insulation Tests of Telegraph
Circuits, G.P.O.

185
D'Arsonval Galvanometer ...

10
Holden's

15
Nalder Brothers'

11
Elliott Brothers'

17
Deflection Method of Measuring
Battery Resistance

71
Deflect on Method of Measuring
Galvanometer Resistance

69
Dial Form of Wheatstone Bridge 33
Tifference of Potential Method for

Determination of Current

Strength
Difference of Potential Equilibrium

Cables, Ends of, Preparation of,
for Testing

82
Straining or “Stressing” of 86
Insulation Resistance of, Measure-
ment of

80
Calibration of Ammeters

183
Calibration of Voltmeters

178
Capacity, Electrostatic, Measure-

ment of
By Direct Deflection Method 125
By Gott's Method

126
By Thomson's Method

127
By Divided Charge Method 128
By Siemens' Diminished Charge
Method

129
By Siemens' Loss of Charge Dis-
charge Method

130
By Siemens' Loss of Charge De-
flection Method

130
Carey Foster's Method of Measur-
ing Low Resistances

97
Carhart's Standard Cell
Cells, Standard (see Batteries)
Chrystal's Standard Ohm

51
Clark's Accumulation Joint Test 171

Method for Determination of
Current Strength

121
Direct Deflection Method for

Determination of Current
Strength

119
Direct Deflection Method for the

Measurement of Electrostatic
Capacity

125
Differentiality, Test for

177
Discharge Key, Lambert's

63
Webb's

64
Method of Joint Testing

173
Divided Charge Method for Measure.

ment of Electrostatic Capacity 128
Dynamometer, Siemens' Electro:... 116

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