provide for three different grades of sensibility. Thus the sensibility with the smallest weight on the knifeedge is a deflection of one division per 50 volts, with the two smaller weights, that is four times the smallest, one division per 100 volts, with all three weights or sixteen times the smallest weight, one division per 200 volts.

The electrostatic voltmeter is graduated as follows. A known difference of potential is obtained by means of a battery of from 50 to 100 cells with a high standard resistance in its circuit. An absolute galvanometer or current balance measures the current in the circuit, and the product of the numerics of the current and the resistance gives that of the difference of potential between the terminals of the latter (see pp. 104, 106). These terminals are connected to the working terminals of the voltmeter, and the deflections noted with the smaller weights on the knife-edge.

For the higher potentials a number of condensers of good insulation are joined in series, and charged by an application of the wires from the terminals of the resistance coil to each condenser in succession from one end of the series to the other. This is done so as to charge each condenser in the series in the same direction, and as the same difference of potential, V say, is produced between the plates of each condenser, the total difference between the extreme plates is nV, if there ben condensers. A convenient large difference of potential can thus be obtained with sufficient accuracy, and being applied to the working terminals of the voltmeter is made to give divisions for a series of different weights hung on the knife-edge. These divisions correspond of course to deflections for known differences of potential with one of the weights on the knife-edge.

The divisions thus obtained are then checked by using three instruments which have been dealt with in this way. They are joined in series and a difference of potential established between the extreme terminals, which is observed also by the third joined across the other two. Thus by a process of successive halving and doubling the scale is filled up.

For smaller differences of potential, ranging from 40 to 800 volts, Sir William Thomson has constructed a multicellular voltmeter (Fig. 28) on the same principle. The indicator consists of a number of equal vanes v, v,..., in shape similar to the needle of the quadrant electrometer, attached horizontally at equal distances apart on a vertical spindle. The spindle passes at its upper end through a small hole at the centre of a shallow circular box, in the bottom of which is the scale of the instrument. This is covered with a glass plate to guard the indicator from currents of air and keep the scale and other interior parts free from dust.

A vertical brass tube t, carries a torsion head h, from which the indicator is suspended by a fine platinum wire w. Between the upper end of the spindle and the wire is interposed a small coach-spring which having sufficient. resilience to allow the spindle to touch a guard-stop, saves the suspension from being broken down by accidental shaking or jolting of the instrument. By turning the torsion head the needle can be adjusted so as to point to zero when no difference of potential is upon its terminals.

Each vane is within a cell similar to that surrounding the vane of the vertical voltmeter but placed in a horizontal position, so that the stationary part of the instrument is a pile of cells or condenser plates e, e,... arranged

vertically above one another. These are all in metallic connection, and between them and the vanes which form the other plate of the condenser the difference of potential to be measured is produced.

Two vertical brass repelling plates d, d are attached to the sole-plate of the instrument, and prevent the indicator from turning too far. The lower end of the spindle passes through a hole in a guide-plate carried by these vertical plates, and is prevented from passing back by a little brass head attached to it below. The spindle hangs free by the suspending wire when the instrument is level, and the vanes are then horizontal, each in a plane halfway between the top and bottom of its cell.

The index, i, is of aluminium and is attached to the top of the spindle so as to indicate on the scale the difference of potential between the indicator and the fixed plates. The instrument is graduated so as to give volts directly by its readings.

The scale can be read in an engine-room from a distance by means of a mirror placed above the instrument at an angle of 45° with the plane of the scale, giving thus a vertical scale by reflection.

When the instrument is to be carried from one place to another, a thumb-screw r, provided below the sole-plate is screwed up so as to take the weight of the spindle off the suspension wire, and clamp the head above referred to against the guide plate.

The instrument is made of four different ranges between the extreme limits of 40 and 800 volts.

The graduation is performed in the following manner. (1) For differences of potential under 140 volts :-A circuit composed of a standard resistance, a standard centiampere balance and a variable resistance joined in

series, is arranged. The multicellular electrometer, properly adjusted, has its terminals joined to the extremities of the standard resistance, and the deflection of its indicator and the current flowing through the balance are both observed. The difference of potential is of course at once got by multiplying the observed current and the standard resistance together. Its value is altered to give different points on the scale by properly changing the variable resistance. Of course the part of the circuit composing the standard resistance and the centiampere balance must be so well insulated as to ensure that all the current which flows through the standard resistance also passes through the centiampere balance.

(2) For differences of potential above 140 volts:-A voltaic pile is used attached by one terminal to one extremity of the standard resistance of the above arrangement, so that the terminals of the multicellular voltmeter attached one to a point in the pile, and the other at the further extremity of the standard resistance, have upon them a difference of potential equal to that between the terminals of the standard resistance plus a constant amount obtained by including the requisite number of couples in the voltaic pile. A previously graduated standard multicellular of the proper grade of sensibility enables this latter difference of potential to be kept constant during the progress of the standardization, while the different points, on the scale of the instrument being standardized, are obtained by varying the difference of potential on the terminals of the standard resistance by means of the variable resistance. Repeating this process the necessary number of times, using as "standard multicellular" first one of the first grade of sensibility (to graduate another of the second grade), then a second

L