A

.

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, dl, 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 somewhat similar to the above, and is represented in diagram.

by Fig. 60. The current to be measured flows from B
to A as before, but in this case a b represents a slide wire
resistance of which S is the contact slider. The galvano-
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 a and
S, which we will call R, or, expressed as a formula,
Es
C =

R

From this it will be seen that the ohmic resistance of

B

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.

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 variable resistance, and Es a standard cell or accumulator, from which a current can be taken, not necessarily as large as C, however. r 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 galvanoEs r rl (R+ r).

meter G, then C =

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

standard condenser of known capacity, either directly or indirectly. Before proceeding further, however, I would deal in brief with a special form of galvanometer, adapted for the measurement of such transient currents as those involved in the electrostatic discharge of a condenser of large capacity, such as a long section of submarine cable, for example. Such a discharge current lasts for a perceptible period of time, whereas the momentary discharges from condensers of low electrostatic capacity are comparatively brief, and may readily be compared by the transient swing of an ordinary Thomson reflecting galvanometer. Nevertheless, as the reader may have occasion to deal with large electrostatic capacities in one or another connection, a knowledge of the construction and principle of what is known as the "ballistic galvanometer" will not come amiss.

The ballistic galvanometer, as commonly constructed, is shown in the accompanying illustration, and is exactly similar to an ordinary Thomson reflecting galvanometer in all respects except the suspended system, the detailed construction of which is represented in diagram in Fig. 62, where a b represents the ordinary aluminium axis, pass

The N.C.S Ballistic Galvanometer, by Nalder Bros.
Stock Winding 1,000 ohms.

ing through the centre of a split cylindrical magnet A, shaped somewhat like a thimble; a transverse section along the line c d is shown at B. These magnets, with their semi-cylindric poles n s, replace the little watchspring magnets in the Thomson instrument, and may be mounted, two with their like poles opposite, in the centre of each coil, and two others immediately outside the coil above and below, as represented in the complete illustration above, or in any other similarly suitable manner, the object of the cylindrical form given to the magnets being that they may offer as little resistance to motion as possible in their passage through the surrounding air. As a matter of fact, in practice, the motion of the needle does not commence until the current causing it has ceased.

When, as is often the case, the number of oscillations made by the needle of a ballistic galvanometer in a given time, is required, it can be obtained by fixing the eye

upon a certain point on the galvanometer scale within the limits of the deflection, and counting the number of times the reflected spot of light passes that point, whilst travelling in the same direction, during the time stated.

For the mathematical and mechanical proofs underlying the action of the ballistic galvanometer, the reader is referred to Kemple's "Handbook of Electrical Testing."

To proceed, however, with the more immediate subject

matter of this section, viz., the determination of electrostatic capacity. For the majority of the tests about to be described in this connection, the ordinary Thomson. reflecting galvanometer is admirably suited, more especially if fitted with the usual damping device to check the more or less irresponsible swings due to the sudden condenser discharges, etc.

The simplest method for the determination of electrostatic capacity is known as the direct deflection method, and is represented diagrammatically in Fig. 63, where G represents a Thomson galvanometer, which may or may not be provided with a shunt across its terminals, according to whether the discharges to be compared are small or large; it is therefore to a great extent dependent upon the battery power E used, but, in any case, a shunt will be found useful, and should be included among the apparatus for the test, although it has been omitted from the figure for the sake of clearness. K represents Lambert's discharge key, previously described and illustrated, whilst C is the standard condenser of known capacity, for which is ultimately substituted the electrostatic capacity or condenser, whose value it is required to determine, such as a length of cable, for example.

The modus operandi is as follows:-K2 is first depressed for a definite period, such as 30 seconds or more, in order to charge the standard condenser C to saturation.

G

K

FIG. 63.