Absorption of Cathode Rays in Aluminum Foil

The experimental method used is a modification of that of Whiddington and of Schonland. Homogeneous rays of accurately known velocity are provided by fitting a Coolidge cathode to a high vacuum tube and exciting it by high tension direct current. The stream of electrons traverses the foil and enters a Faraday cylinder connected to a galvanometer. The foil may be swung aside to permit measurements of the direct beam. One of three foils of rolled aluminium.00031 cm thick were studied with voltages 18 to 52 kv. Up to the vicinity of the range limit, Lenard's equation, $\frac{I}{I_0}=e^{-\alpha x}$ is found to hold, where $x$ is the thickness. Within this range of velocities, 8.09 to 12.23×${10}^9$ cm/sec., the values of $\alpha$ satisfy the approximate relation derived by J. J. Thomson, $\alpha v^4=.\mathrm{}268\mathrm{} \left({10}^{44}\right)$, where $v$ is the mean of the velocities before and after penetrating the foil.