Page:Radio-activity.djvu/123

 where b is the internal diameter of D, a the external diameter of C, and l the length of the tubes.

The following method can be used in some cases with advantage. While a testing vessel is in connection with the electrometer, a sample of uranium is placed on the lower plate A. Let d_{2} and d_{1} be the number of divisions passed over per second by the needle with and without the standard capacity in connection.

Then    (C + C_{1})/C = d_{1}/d_{2}, and     C = C_{1} d_{2}/(d_{1} - d_{2}).

This method has the advantage that the relative capacities are expressed in terms of the motion of the needle under the actual conditions of measurement.

69. Steady deflection method. The methods of measurement previously described depend upon the rate of angular movement of a suspended gold-leaf or of an electrometer needle. The galvanometer can only be employed for measurements with intensely active matter. A need, however, has long been felt for a method in which ordinary ionization currents can be measured by means of a steady deflection of an electrometer needle. This is especially the case, where measurements have to be made with active substances whose activity alters rapidly in the course of a few minutes.

This can obviously be secured if the electrometer system (one pair of quadrants being earthed) is connected to earth through a suitable high resistance. A steady deflection of the electrometer needle will be obtained when the rate of supply of electricity to the electrometer system is balanced by the loss due to conduction through the resistance. If the high resistance obeys Ohm's law, the deflection should be proportional to the ionization current to be measured.

A simple calculation shows that the resistance required is very great. Suppose, for example, that a current is to be measured corresponding to a rate of movement of the needle of 5 divisions per second, with a sensibility of 1000 divisions per volt, and where