of ·0053 cms.—a thickness just sufficient to absorb the α rays—and made the insulated electrode in a cylindrical metal vessel which was rapidly exhausted to a low pressure. The current in the two directions was measured at intervals by an electrometer, and, as we have seen in section 93, the algebraic sum of these currents is proportional to ne, where n is the number of β particles expelled per second from the lead rod, and e the charge on each particle. The activity of the radium C decayed with the time, but, from the known curve of decay, the results could be corrected in terms of the initial value immediately after the rod was removed from the emanation.
Taking into account that half of the β particles emitted by the active deposit were absorbed in the radium itself, and reckoning the charge on the β particle as 1·13 × 10^{-19} coulombs, two separate experiments gave 7·6 × 10^{10} and 7·0 × 10^{10} as the total number of β particles expelled per second from one gram of radium. Taking the mean value, we may conclude that the total number of β particles expelled per second from one gram of radium in radio-active equilibrium is about 7·3 × 10^{10}.
The total number of α particles expelled from one gram of radium at its minimum activity has been shown to be 6·2 × 10^{10} (section 93). The approximate agreement between these numbers is a strong indication of the correctness of the theoretical views previously discussed. It is to be expected that the number of β particles, deduced in this way, will be somewhat greater than the true value, since the β particles give rise to a secondary radiation consisting also of negatively charged particles moving at a high speed. These secondary β particles, arising from the impact of the β particles on the lead, will pass through the aluminium screen and add their effect to the primary β rays.
The results, however, indicate that four α particles are expelled from radium in radio-active equilibrium for each β particle and thus confirm the theory of successive changes.
