CH. iv.j BY CATHODE RAYS 33 



particles of some kind, in extremely rapid motion. 

 That they are in motion must be granted from the 

 fact of their bombardment driving mills, heating 

 platinum, and the like ; and in order to show that 

 they are charged, the most direct plan is to catch 

 them in a hollow vessel connected with an electro- 

 scope, as Perrin did ; but another plan is to show 

 that they have the properties of an electric current. 

 If they are charged while in motion they constitute a 

 current, on Maxwell's theory, and therefore should 

 be able either to deflect a magnet or to be deflected 

 by it; and here comes one of the most simple and 



Electroscope 



FIG. 2. Simplest form of Perrin's apparatus for proving that cathode 

 rays carry a negative charge. The rays from a pass through an earthed 

 screen b into a hollow or Faraday vessel c. 



important experiments in physics at the present 

 time. A definite form of some old experiments 

 foreshadowed by Pliicker (1862), and developed by 

 Hittorf, Goldstein, and many other vacuum tube 

 observers, was arranged by Crookes in 1879, when 

 he made the track of the rays visibly luminous by 

 passing a pencil of them through a slit and letting 

 them graze along the surface of a film of mica covered 

 with phosphorescent powder, and when he then 

 brought near them a common horseshoe magnet. 

 When this is done the track of the rays is at once 

 seen to be curved ; showing that it is not a beam 

 of light we are looking at, but a torrent of charged 

 particles ; since they behave like an electric current 

 and are deflected by a magnet. It is ultimately the 

 very same phenomenon as can be observed with 

 difficulty, owing to its smallness, when a current 



L.E. 



