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effective range of the instrument according as to whether the current to be measured is large or small. The fixed coil, which corresponds with a b c D, Fig. 58, is supported on a suitable stand as shown, and is surrounded by a stout wire rectangle, lying in a plane at right angles to it, and suspended at its upper extremity from a thumb

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screw by means of a thin fibre running through the centre of a delicate spiral spring, which is also rigidly attached both to the rectangle and to the aforementioned thumb-screw. Superimposed upon these intersecting coils is a horizontal graduated dial, the periphery of which is divided into degrees, and around the outer edge of which, between the limits of two stops placed a short distance apart, and embracing at their centre the zero, or 0° point of the scale, plies an index finger, also rigidly attached to the movable coil. A second radial index attached to the thumb-screw also indicates upon the circumference of the scale, whilst three levelling screws and a suitable level or plumb-line complete the apparatus. Electrical connection with the movable coil or rectangle is made by means of two mercury cups, into which its lower extremities dip.

The mode of usage is

as follows:-The apparatus having been set up and levelled until the movable coil is free to move in either direction, but remains stationary with both indices at zero, the current to be measured is passed through by way of one pair of terminals or the other, according to its probable value, and a deflection of the movable coil until checked by one of the stops is the result. The thumb-screw at the top is then turned in the opposite direction until the pointer or index attached to the movable rectangle is brought back to zero by the consequent torsion of the spiral spring. The angle through which the radial pointer has been turned to secure this result is then read off upon the horizontal dial, and the current passing is indicated upon a table of degrees and corresponding currents specially prepared for the instrument. This table is constructed by the manufacturers in the instance by passing a current of known value through the instrument, and noting the number of degrees of torsion required to bring the movable index to zero. When this has been ascertained, the remainder of the table can be deduced by simple rule of three.

As may readily be imagined, the electro-dynamometer is most accurate when used for large currents, which require a considerable degree of torsion to counteract their deflective effects upon the movable rectangle, as in such cases the percentage of error is very small compared with that attendant upon the measurement of correspondingly small values.

The direct deflection method of current measurement is a comparatively simple one, and depends for its accuracy on the corresponding definition by the observer's eye, of the galvanometer readings. It involves the employment of a low resistance galvanometer, the resistance of which is known, a standard cell or accumulator from which a current equivalent to that to be measured can be taken without disturbing its constancy, and a variable resistance of sufficient dimensions to carry the current under test without appreciable heating. The resistance of the standard cell must either be known, or of so small a value as to be negligible in calculating subsequent results.

The galvanometer is first joined up in the circuit through which is flowing the current which it is required to measure, and its deflection duly noted. It is then disconnected from the circuit, and inserted in simple series with the standard cell and variable resistance, and the latter is adjusted until the same deflection is obtained on the galvanometer as before, which is due evidence of the passage of an equivalent current. Then by Ohm's law, the current in each case is equal to the electromotive force of the standard cell in volts, divided by the total resistance of the latter circuit in ohms. If unknown, the E.M.F. of the standard cell can be ascertained by one of the methods already described for the determination of electromotive force.

Expressed as a formula, let E be the E.M.F. of the
standard cell,
Let C ba the current which it is required to measure.

Rg resistance of the galvanometer.
R variable resistance (in circuit).
Rc resistance of the standard cell (if

required) Then C =

Rg + R + Rc The difference of potential deflection method for the determination of current strength is indicated in Fig. 59, where a b is a low resistance introduced into the path of

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the current to be measured, which is flowing from A to B, as indicated by the arrow heads, G is a high resistance Thomson galvanometer, and r an auxiliary resistance, also of high value, the combined resistance of G and r being such that they do not materially influence the value of

the current to be measured, which we will call C, by
their introduction in derived circuit, as shown.

The galvanometer being connected to the points a and
b, as indicated in the figure, a deflection d results from
the difference of potential between these points ; this
deflection is noted, and the galvanometer and its at-
tendant resistance are then disconnected from a b, and
connected instead to the terminals of a standard cell,
the electromotive.force, Es, of which is known. A second
deflection, di, is thus obtained, then the current to be

Es d
measured, C= where R is the value of the low

R dl
resistance a b in ohms or fractions of an ohm.

The difference of potential equilibrium method is some-
what similar to the above, and is represented in diagram

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by Fig. 60. The current to be measured flows from B
to A as before, but in this case a b represents a slide wira
resistance of which S is the contact slider. The galva 20-
meter G and standard cell Es are connected as shown,
such that the current from the standard cell Es tends to
oppose that to be measured, which, as before stated,
is flowing from B to A in the direction indicated by the
arrow heads. The slider S is adjusted until no deflection
results upon the galvanometer G, then the required cur-
rent C is equal to the E.M.F. of the standard cell Es,
divided by the resistance of the slide wire between u azid
S, which we will call R, or, expressed as a formula,

C =

From this it will be seen that the ohmic resistance of

any given length of the slide wire, in terms of the divisions on its scale, must be known.

Kempe's bridge method is a modification of that devised by Major Cardew, R.E., but since the latter method involves the use of a specially constructed galvanometer, we will not touch upon it here.

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Kempe's method is indicated in Fig. 61, where the current to be measured, C, flows from B to A, as before. R is a vari resistance, and a standard cell or accumulator, from which a current can be taken, not necessarily as large as C, however. p and rl are also resistances of fixed value; the resistance of r as compared with rl determines the value of the current which will be taken from the standard cell Es as compared with C, the current to be measured.

The connections being as shown in the figure, R is adjusted until no deflection results upon the galvanometer G, then C=

rl (R + r). From the determination of current strength, we next pass on to

(10) The Measurement of Electrostatic Capacity, which usually involves a comparison of the electrostatic discharge from the condenser under test with that from a

Es r


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