The fixed inspectional scale shows, approximately enough for most purposes, the strength of the current; the notches in the top of the aluminium scale show the precise position of the weight corresponding to each of the numbered divisions on the fixed scale, which practically annuls error of parallax due to the position of the eye. When the pointer is not exactly below one of the notches corresponding to integral divisions of the inspectional scale, the proportion of the space on each side, to the space between two divisions, may be estimated inspectionally with accuracy enough for almost all practical purposes. Thus we may readily read off 34'2 or 34'7 by estimation, with little chance of being wrong by one in the decimal place. But when the utmost accuracy is required, the reading on the fine scale of equal divisions must be taken, and the strength of the current calculated by the aid of a table of doubled square roots. Thus, for example, if the reading is 292, we find 34'18, or say 34'2, as the true scale reading for the strength of current; or, again, if the balancing position of the pointer be 301 on the fine scale, we find 3470 as the true reading of the inspectional scale. The centi-ampère balance, with a thermometer to test the temperature of the rings, and with platinoid resistances up to 1600 ohms, serves to measure potentials from 10 to 400 volts, the following being the constants when so used :--

If the second pair of weights is used, the constants will be double those given above.

173. Adjustment of the Ealances.--The instrument should be levelled, in accordance with the indications of the attached spirit-level, by means of the levelling screws on which the sole plate of the instrument stands.

1 Including the resistance of the instrument, which is about 50 ohms.

In the centi- and deci-ampère balances, the beam can be lifted off its supporting ligaments by turning a handle attached to a shaft passing under the sole-plate of the instrument.

This shaft carries an eccentric, on the edge of which rests the lower end of a vertical rod, which is fixed at its upper end to a tripod lifter. When the instrument is to be removed by hand from place to place, the lifter should be raised; but when it is fixed up for regular use, it is advisable to keep the beam always hanging on the ligaments.

A set of four sliding weights, of which the carriage constitutes one, is supplied with each instrument. The carriage is fitted with an index to point to the movable scale, and is intended to remain always on the rail. One or other of the weights is to be placed on the carriage in such a way that the small hole and slot in the weights pass over the conical pins. The weights are moved by means of a slider, which slides on a rail fixed to the sole-plate of the instrument, and carries a pendant with a vertical arm intended to pass up through the rectangular recess in front of the weight and carriage. The slider and weight are shown in position in the figure. The slider is moved by silk cords, which pass out at the ends of the glass case. When the cords are not being pulled for shifting the weight, their ends should be left free so that the pendant may hang clear of the weight.

When a weight is to be placed on or removed from the carriage, the slider should be drawn forward at the top until it is clear of the weight, and then pushed to one side until the weight is adjusted, when it may be replaced in position in a similar

manner.

Cylindrical counterpoise weights, with a cross-bar passed through them, are supplied for the purpose of balancing the sliding weights when they are placed at the zero of the scale. The sliding weight should be placed so that the index of the carriage points to the zero of the scale, and the proper counterpoise weight should be placed in the trough, fixed to the righthand end of the beam, with its cross-bar passing through the hole in the bottom of the trough. The flag, which is attached to the cross trunnion of the beam, should then be turned by

means of the handle projecting from under the sole-plate until the index on the end of the movable scale points to the middle one of the fine black lines on the fixed scale opposite to it. Care must be taken when making this adjustment that the fork which moves the flag is not left in contact with it, as this would impede the free swing of the beam. The fork should be turned back a little after each adjustment of the flag, and, when the flag is being adjusted, it is better to watch the flag itself, and make successive small adjustments until the beam stands at zero, than to make successive trials by pushing the handle round while watching the position of the index.

If the ligament has stretched since the instrument was standardized, the index at one end of the movable scale will be found to be below the middle line on its vertical scale, when the index at the other end is correctly pointing to its zero position. The error so introduced would be a small one, but it may easily be put right by slightly loosening the movable beam to the baseplate, and raising it by slipping one or two thicknesses of paper below it until the indexes simultaneously point to their zero position.

In using the centi-ampère balance as a voltmeter when great accuracy is required, care must be taken that the effect of change of temperature in changing the resistance of the coils of the instrument, and of the external resistance coils is allowed for; and in this use of the instrument it is advisable to employ currents such as can be measured by the lightest weight on the beam. When the instrument is to be used as a voltmeter, four resistances are provided, three of which are 400 ohms, and the fourth is less than 400 ohms, by the resistance of the coils of the instrument at a certain specified temperature. The smallest resistance is intended to be included by itself in the circuit when the lowest potentials are being measured, and in series with one or more of the others when the potential is so high as to give a stronger current than can be measured with the lightest weight on the beam. The correction for temperature is, for the copper coils of the balance, about 0.38% per 1o C., and for the platinoid resistances, about o'024% per 1° C.

SIEMENS' ELECTRO-DYNAMOMETER.

174. A most useful piece of apparatus in the laboratory for the measurement of currents is the Siemens' electro-dynamometer. It also partakes of the nature of a secondary standard current measurer, inasmuch as it is independent of variations of the earth's magnetism or of the magnetism of permanent magnets, the controlling force being supplied by the torsion of a spiral spring, the constant of which is determined once for all, and, provided the instrument is carefully used, will remain unaltered for years. The instrument is shown in Fig. 87, and consists essentially of two coils-one fixed to the framework of the instrument, and the other suspended by a silk thread so that it hangs with its axis at right angles to that of the other coil. To the upper end of the suspended coil is also fixed one end of the spiral spring, the other end of which is attached to a torsion head, which has a pointer moving over a circular scale divided into degrees. A pointer is also attached to the movable coil, which is free to move between two stops on either side of the scale zero. The free ends of the suspended coil dip into mercury cups, the connections being made so that the fixed and movable coils are in series. In most forms of this instrument there are two fixed coils-one consisting of a few turns of thick wire, and the other of many turns of finer wire, thus enabling two degrees of sensibility to be obtained. The central terminal (3) is common to both coils, terminals I and 3 are those of the suspended coil in series with the thin fixed coil, while terminals 2 and 3 are those of the suspended coil in series with the thick fixed coil. shown in the figure, actuated by a screw at the instrument, can be released when the instrument is not in use, and supports the weight of the suspended coil.

A spring not back of the

In setting the instrument up, it must be placed so that when the suspended coil swings free its axis is in the magnetic meridian, so that the earth's magnetism will not produce any deflecting effect on it. The instrument is levelled until the coil swings free, and its pointer is at the zero on the scale, the torsion head pointer also being at zero. The winding of