Loss of Velocity of Cathode Rays in Matter

Abstract

Loss of energy of cathode rays in passing through metallic foils.—Whiddington's apparatus was modified to provide a bundle of homogeneous rays by fitting a Coolidge cathode into a high vacuum tube and exciting it by rectified high tension current of accurately known voltage. After traversing the metal foil, the electron beam is deflected by the magnetic field of a solenoid and is spread into a broad band of which a small section, deflected about 30°, passes through a fixed slit into a Faraday cylinder connected to an electroscope. By varying the solenoid current, energy distribution curves were obtained, and the most probable energy loss for each case was determined from the position of the maximum. Rolled foils of $\mathrm{Ag}$, $\mathrm{Al}$, $\mathrm{Au}$, $\mathrm{Be}$ and $\mathrm{Cu}$ were studied, for 25 to 51 kv, giving rays of 9 to 12.6×${10}^9$ cm/sec. velocity. The results agree with the velocity formula of J. J. Thomson, ${v_0}^4-{v_x}^4=ax$, where $x$ is the thickness, and $a$ is a constant which comes out proportional to the density of the metal so that $\frac{a}{\rho }=5.05\mathrm{}\times {10}^{42}$ approximately. In the corresponding voltage formula ${V_0}^2-{V_x}^2=bx$, $\frac{b}{\rho }=.\mathrm{}40\mathrm{}\times {10}^{12}$.