. Radio-activity . trength of currents by electrometers, it is always necessary todetermine the sensibility of the instrument and the capacity of theelectrometer and the apparatus attached thereto. By means of thequartz piezo-electrique devised by the brothers MM. J. and P. Curie*,measurements of the current can be made with rapidity andaccuracy over a wide range. These measurements are quite inde-pendent of the capacity of the electrometer and external circuit. ■■• J. and p. Curie, C. R. 91, pp. 38 and 294, 1880. See also Friedel andJ. Curie, C. R. 96, pp. 1262 and 1389, 1883> and Lord Kel
. Radio-activity . trength of currents by electrometers, it is always necessary todetermine the sensibility of the instrument and the capacity of theelectrometer and the apparatus attached thereto. By means of thequartz piezo-electrique devised by the brothers MM. J. and P. Curie*,measurements of the current can be made with rapidity andaccuracy over a wide range. These measurements are quite inde-pendent of the capacity of the electrometer and external circuit. ■■• J. and p. Curie, C. R. 91, pp. 38 and 294, 1880. See also Friedel andJ. Curie, C. R. 96, pp. 1262 and 1389, 1883> and Lord Kelvin, Phil. Mag. 36,pp. 331, 342, 384, 414, 453, 1893. 106 METHODS OF MEASUKEMENT [CH. The essential part of this instrument consists of a plate ofquartz which is cut in a special manner. When this plate isplaced under tension, there is a liberation of electricity equal inamount but opposite in sign on the two sides of the plate. Theplate of quartz AB (Fig. 21) is hung vertically and weights are To Support Ean. Earth To Weight Fig. 21. added to the lower end. The plate is cut so that the optic axis ofthe crystal is horizontal and at right angles to the jDlane of thepaper. The two faces A and B are normal to one of the binary axes(or electrical axes) of the crystal. The tension must be applied ina direction normal to the optic and electric axes. The two facesA and B are silvered, but the main portion of the plate is electricallyinsulated by removing a narrow strip of the silvering near the upperand lower ends of the plate. One side of the plate is connected withthe electrometer and with the conductor, the rate of leak of whichis to be measured. The quantity of electricity set free on one faceof the plate is accurately given by Q = 063 ^ F, 0 Ill] METHODS OF MEASUREMENT 107 whero L is the length of the insulated portion of the plate, b thethickness AB, and F the weight attached in kilogrammes. Q isthen given in electrostatic units. Suppose, for example, that it is required t
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