THE GROWTH OF PHYSICAL IDEAS 107 



When Thomson showed later that the cathode stream irt 

 an exhausted tube is a stream of electrons, it was realized that 

 electrons falling on a target, such as the end of the tube, cause 

 x-rays to be emitted, and targets were then placed in the tubes 

 so that the cathode stream was focused on them. The early 

 x-ray tubes had a hemispherical cathode and a target or anti- 

 cathode, as it was called, made of platinum. These tubeS' 

 were exhausted to a high vacuum, but not too high, as other-^ 

 wise the current would not pass. 



In 1884 J. J. Thomson became the Cavendish professof of 

 physics at the University of Cambridge, and as his first major 

 piece of work he started to study the cathode rays to deter* 

 mine whether they were of a wave nature, similar to light, 

 or whether, as Crookes believed, they consisted of particles 

 carrying a charge of electricity. Thomson wrote many years 

 afterward: * 



I had for a long time been convinced that these rays 

 were charged particles, but it was some time before I had 

 any suspicion that they were anything but charged atoms. 

 My first doubts as to this being the case arose when I meas- 

 ured the deflection of the rays by a magnet, for this was far 

 greater than I could account for by any hypothesis which 

 seemed at all reasonable if the particles had a mass at all 

 approaching that of the hydrogen atom, the smallest then 

 known. 



By measuring both the magnetic deviation and the total 

 energy of the rays, using a thermocouple to find their heat- 

 ing effect, Thomson was able to calculate the velocity of the 

 rays and the ratio of the mass of the particles to the electric 

 charge. The conclusion showed that the velocity was enor- 

 mously high— 5 per cent of the velocity of light, much higher 

 than could be expected for any molecule or atom— and that 

 the ratio of the mass to the charge was much less than would 

 be possible for hydrogen atoms. If the rays consisted of elec- 

 trified particles, the particles were something quite new to 



* Lord Rayleigh, Sir J. J. Thomson, p. 80, Cambridge University 

 Press, 1942. 



