Natures of various Electric Radiations. 



447 



is not clear that Marx really measured the velocity o£ a 

 radiation causing the emission of high-speed electrons, which 

 is the characteristic feature of X rays : all that he showed 

 was that the bundle of X rays contained radiation capable 

 of exciting 8 rays. To see this it is necessary to consider 

 briefly the details of the experiment. 



An electric pulse is made to travel along a wire, W, as 

 shown in the accompanying sketch. When it reaches the 



£arth 



cathode, C, cathode rays are driven against the anode, A, and 

 X rays are given out, some of which travel towards the 

 saucer-shaped electrode, B. At the focus of B is a small 

 Faraday cylinder, F, connected to an electrometer, E. A 

 small impulse is derived from the wire, W, by electrostatic 

 induction at D, and travels down to B. If the various 

 distances and wire-lengths are properly adjusted, so that the 

 X rays arrive at B at the same moment as the derived im- 

 pulse, electrons are liberated at B by the rays, and guided 

 by the impulse into the cylinder, F, and thence to the elec- 

 trometer. If now the distance of the X-ray bulb from B 

 is altered, say, by an increase of 10 cm., the wire from D to 

 B has to be lengthened by 10 cm. Thus, according to Marx, 

 the X rays travel with the same velocity as the impulse in 

 the wire, and therefore with the velocity of light. 



But it is to be remembered that the electrons which are 

 liberated by X rays have an initial velocity averaging about 



2H2 



