raised. The disinsulator mentioned above (p. 128) is used to free the quadrants connected with L from the induced charge which they generally receive when L is raised. Either of these grades of sensibility may of course also be varied by increasing the distance of the fibres apart. The quadrant electrometer can be made to give results in absolute measure by determining the constant C' of equation (3), by which the deflection must be multiplied to give the difference V1 - V2 This can be done by observing the deflection produced by a battery of electromotive force of convenient amount, determined by direct measurement with an absolute electrometer or by connecting the electrodes of the instrument to two points of a circuit the difference of potential between which is at the same time measured by a standard balance used as a voltmeter (see p. 104). Different such electromotive forces may be employed to give deflection of different amounts and thus give a kind of calibration of the scale to avoid error from non-fulfilment of condition of proportionality of deflection to difference of potential. The quadrant electrometer may also be used idiostatically for the measurement of differences of potential of not less than about 30 volts (p. 69). When it is so used the jar is left uncharged, the charging-rod is brought into contact with the inner coating of the jar, and joined by a wire with one of the main electrodes, so as to connect the needle to one pair of quadrants. The other pair of quadrants is either insulated or connected to the case of the instrument. The instrument thus becomes a condenser, one plate of which is movable, and by its change of position alters the electrostatic capacity of the condenser. The two main electrodes are connected with the conductors, the difference of potential between which it is desired to measure. A lower grade of sensibility can be obtained by connecting the needle through the charging-rod to the electrode R, and using the induction-plate instead of the pair of quadrants connected with L, which are insulated by raising their electrode. When the instrument is thus used idiostatically V in equation (2) above becomes equal to V1, and instead of (2) we have that is, the deflection is proportional to the square of the difference of potential and therefore independent of the sign of that difference. It is to the left or right according to the electrode connected to the needle. This independence of sign in the deflection renders the instrument thus used applicable to the determination of difference of potentials in the circuits of alternating dynamo- or magneto-electric generators. (See Chap. IX.) The quadrant electrometer has been modified by different makers. In a form made in Paris for M. Mascart, the needle is kept at a constant potential by being connected to the positive pole of a dry pile, the negative pole of which is connected to the case, and the replenisher is dispensed with. In another form devised by Prof. Edelmann of Munich, and suitable for some purposes as a lectureroom instrument, the quadrants are longitudinal segments of a somewhat long vertical cylinder, and the needle consists of two coaxial cylindric bars connected by a cross-frame, and suspended by means of a bifilar. A glass vessel below contains strong sulphuric acid in which dips a vane carried by a platinum wire attached to the needle. For practical work Sir William Thomson has lately constructed a form of electrometer to be used idiostatically, and has called it an electrostatic voltmeter. It is represented in Fig. 27, and may be described as an air condenser, one plate of which, corresponding to the needle of the quadrant electrometer, is pivoted on a horizontal knife-edge working on the bottoms of rounded V-grooves cut in the supporting pieces. This plate by its motion alters the electrostatic capacity of the condenser. The fixed plate consists of two brass plates in metallic connection, each of the form of a double sector of a circle, which are placed accurately parallel to one another, with the movable plate between them as shown in the figure. The upper end of the movable plate is prolonged by a fine pointer which moves along a circular scale, the centre of which is in the axis. The fixed plates are insulated from the case of the instrument; the needle is uninsulated. Contact is made with the plates by insulated terminals fixed outside the case. The two shown on the lefthand side in the figure belong to the fixed plate, and a similar pair on the right-hand side are in connection with the movable plate through the supporting Vgroove and knife-edge. The terminals of each pair are connected by a safety arc of fine copper wire contained within a U-shaped glass tube suspended from them and the terminals in front in the diagram which are separated from the plates by the arcs of wire are alone used for connecting to the conductors, or two points of an electric circuit, the difference of potential between which is to be measured, and are therefore called the working terminals. The instrument is enclosed in a wooden case to diminish the risk of injury to incautious or uninstructed persons who might chance to touch its terminals. When a difference of potential is established between the fixed and movable plates the latter moves so as to increase the electrostatic capacity of the condenser, and the couple acting on the movable plate in any given position is, as in the quadrant electrometer when used idiostatically, proportional to the square of the difference of potential. This couple is balanced by that due to a small weight hung on the knife-edge at the lower end of the movable plate. The scale is graduated from o° to 60° so that the successive divisions represent equal differences of potential. Three different weights, 325, 97*5, 390 milligrammes respectively, are sent with the instrument to |