Page:Radio-activity.djvu/125

 ionization between the plates AB, and a steady deflection was obtained when the rate of supply was equal to the rate of discharge.

This air resistance obeyed Ohm's law over a considerable range, i.e. the steady deflection was proportional to the current. It is advisable, in such an arrangement, to test whether the deflection is proportional to the ionization current over the range required for measurement. This can readily be done by the use of a number of metal vessels filled with a constant radio-active substance like uranium oxide. The effect of these, when placed in the testing vessel, can be tested separately and in groups, and in this way the scale can be calibrated accurately.

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 [Greek: beta] and [Greek: gamma] 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 , 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.