The plates _AB_ were placed inside a closed vessel to avoid air currents. The contact difference of potential between the plates _AB_, which shows itself by a steady deflection when no radio-active matter is present in _CD_, was for the most part eliminated by covering the surface of the plates _A_ and _B_ with very thin aluminium foil. This method proved very accurate and convenient for measurement of rapid changes in activity, and possesses many advantages over the ordinary rate-method of use of an electrometer. A thin layer of radium of moderate activity would probably serve in place of the radio-tellurium, but the emanation and the β and γ rays emitted from it would be a possible source of disturbance to the measurements. The deflection of the electrometer needle in this arrangement is independent of the capacity of the electrometer system, and thus comparative measurements of current can be made without the necessity of determining the capacity in each case. =70.= =Quartz piezo-electrique.= In measurements of the strength of currents by electrometers, it is always necessary to determine the sensibility of the instrument and the capacity of the electrometer and the apparatus attached thereto. By means of the quartz piezo-electrique devised by the brothers MM. J. and P. Curie[110], measurements of the current can be made with rapidity and accuracy over a wide range. These measurements are quite independent of the capacity of the electrometer and external circuit. The essential part of this instrument consists of a plate of quartz which is cut in a special manner. When this plate is placed under tension, there is a liberation of electricity equal in amount but opposite in sign on the two sides of the plate. The plate of quartz _AB_ (Fig. 21) is hung vertically and weights are added to the lower end. The plate is cut so that the optic axis of the crystal is horizontal and at right angles to the plane of the paper. [Illustration: Fig. 21.] 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 in a direction normal to the optic and electric axes. The two faces _A_ and _B_ are silvered, but the main portion of the plate is electrically insulated by removing a narrow strip of the silvering near the upper and lower ends of the plate. One side of the plate is connected with the electrometer and with the conductor, the rate of leak of which is to be measured. The quantity of electricity set free on one face of the plate is accurately given by _L_ _Q_ = ·063 ---- _F_ _b_ where _L_ is the length of the insulated portion of the plate, _b_ the thickness _AB_, and _F_ the weight attached in kilogrammes. _Q_ is then given in electrostatic units.