Composite Balance.

(1) This instrument is similar in form to the centi- and deciampere balances, but the pair of fixed coils at one end of the beam are made of a rope of insulated wires similar to that used for the coils of the hekto-ampere balance. Separate electrodes are provided for the rope coils, and for the fine wire coils. A switch which allows the movable coils either to be included in the circuit by themselves or in series with the fixed fine wire coils is attached to the under side of the sole-plate of the instrument. When the handle of the switch is turned to " Watt," the movable coils alone are in the circuit; but when the handle is turned to "Volt," both the movable and the fixed fine wire coils are in the circuit.

(2) The composite balance (Fig. 152) can be used as a hektoampere balance, or as a Wattmeter, or as a voltmeter, by following the instructions given below.

To enable the composite balance to be used as a direct-reading Wattmeter or voltmeter, a separate anti-inductive resistance of platinoid wire, subdivided into four coils, is usually supplied. The first coil is equal to the resistance of the fixed fine wire coils, and is intended to be included in the circuit of the movable coils when the instrument is used as a Wattmeter. The second coil is arranged to make up 200 ohms with the resistance of the fine wire movable and fixed coils. The third coil is 200 ohms and the fourth is 400 ohms.

It is not advisable that the current through these resistances should be allowed to exceed 0.5 ampere.

INSTRUCTIONS FOR THE USE OF THE COMPOSITE BALANCE.

(3) The balance should be levelled and the stop screws turned back out of contact with the cross trunnion and the front plate of the beam so as to leave it free to oscillate.

(4) To use the instrument as a centi-ampere meter or as a voltmeter the switch is turned to "Volt," and one or other of the weights marked VW1, VW, VW3, used. The current flowing through the instrument is then to be calculated from the constant given in the certificate sent with the instrument.

1

The volts on the terminals are calculated from the current in amperes and the resistance in ohms (including the anti-inductive resistance, if any) in circuit. If V be volts, C current, and R resistance,

V=CR.

The anti-inductive resistance is arranged so that the instrument reads a round number of volts per division.

TABLE IX.

CONSTANT OF COMPOSITE BALANCE WHEN USED AS A VOLTMETER.

1 Including the resistance of the instrument, which is about 30 ohms.

If the second pair of weights (Sledge+ VW2) be used the constants will be double of those noted above.

(5) To use the instrument as a hekto-ampere meter the switch is turned to "Watt" and the thick wire coils inserted in the current circuit in such a way that the right-hand end of the beam is repelled up. Either the sledge alone or the weight

marked WW is to be used in this case. A measured current is then passed through the suspended coils, and the constants given in the certificate for the balance used in this way are calculated on the assumption that this current is, as there stated, 0.25 ampere, but any other current which is convenient in the circumstances may be used. The current through the suspended coils may be measured by means of the instrument itself arranged for the measurement of volts. This may be done by first mea

suring the current which the difference of potential between the supply conductors of an electrical installation, or between the poles of a battery, causes to flow through the coils of the instrument and its external resistance, and then turning the switch to Watt," and at the same time introducing a resistance into the circuit equal to the resistance of the fixed coils.

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CONSTANT OF COMPOSITE BALANCE WHEN USED AS A HEKTO-
AMPERE BALANCE.

1 With 0.25 ampere through movable coils.

N.B.-The constants vary inversely as the current through the fine wire coils.

(6) To use the instrument as a Wattmeter, one terminal of the fine wire coils is joined to one end of the anti-inductive resistance and the other terminal to one of the leads; the other end of the resistance being joined to the other lead. The thick wire coils are inserted in the main circuit as described in sect. 5 above. With the instrument thus joined up, the current through the suspended coils and the E.M.F. between the leads may be obtained by the operations described in sect. 4 above, since the presence of the thick wire coil in the circuit causes no appreciable error : or the E.M.F. may be taken from the electrostatic voltmeter used on the circuit, and from its indications the current in the suspended coil circuit calculated. The watts are then to be calculated from the E.M.F. on the leads and the current through the thick wire coils by the formula

where c is the current in the suspended coil circuit, C the current in the thick wire coils, and R the resistance in the circuit.

The weights sent out with the instrument are arranged to give a round number of Watts per division of the scale with a known anti-inductive resistance in series with the fine wire movable

coils.

TABLE X.

CONSTANT OF COMPOSITE BALANCE WHEN USED AS A WATTMETER.

1 Including resistance of movable coils, which is about 12 ohms.

Adjustable Magneto-static Current Meter.

(1) The magneto-static current meter (Fig. 153) consists essentially of a small steel magnet or system of magnets suspended in the centre of a uniform field of force due to two coils, each having one or more turns of copper ribbon or wire, and also under the directive influence of two systems of powerful steel magnets.

(2) The suspended system of magnets is attached to one end of a vertical shaft passing down centrally through an opening in the sole-plate of the instrument from an indicating needle, which is supported by a jewelled cap resting upon an iridium point.

(3) The two systems of directive magnets are circular in form, and each ring is composed of two semicircular magnets placed in a brass cylindrical frame with their similar poles together. Each system is securely fixed to a circular brass frame, which fits on to the cylindrical case of the instrument in such a manner that the systems are capable of being turned round, together or separately, as explained below.

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(4) The instrument has a tangent scale," which is adjusted in its position before the instrument is sent out, so that the needle indicates equal differences of readings for equal differences of current. The scale consists of a hundred divisions, and for most purposes it is convenient to set the field magnets in such a position