Energy conveyed by cathode rays
One of Sir William Crookes’s tubes contained a small paddle wheel running on rails inside it and having mica vanes (Fig. 44.6). The two electrodes are arranged in such a
way that cathode rays impinge only on vanes above the axis. On passing the discharge the paddle wheel rotates towards the anode. It appears to be pushed along by the cathode rays. Actually, this force is not directly due to the momentum of the electrons but is really a secondary effect. When the electrons strike the vanes their energy becomes transferred to internal energy, thereby raising the temperature of the vanes’ surface. Gas molecules in the tube receive heat energy when they come into contact with the warm surface and rebound with high velocity, thus imparting an equal and opposite momentum to the vanes. The force of reaction thus set up causes the paddle wheel to rotate.
More is said about what goes on inside an electric discharge tube on page 516, followed by a description of the more recent hot-cathode types of discharge tube.