PHYSICS 331 



done, that I ask your indulgence while I try for a few minutes, frag- 

 mentarily, to describe one or two fundamental experiments. 



Cathode rays are invisible, but many substances — fortunately glass 

 is of the number — shine with a bright phosphorescent light when placed 

 in the path of the rays. By this means it was early discovered that 

 cathode rays travel in straight lines which always leave the cathode 

 making right angles with the metal surface from which they depart. 

 It is possible, therefore, to make the cathode concave or saucer-shaped 

 and thus bring the rays to a focus at some point in the tube. If cathode 

 rays are thus focussed upon the blades of a very delicate paddle wheel 

 which rotates easily upon an axis, the wheel is set revolving as if struck 

 by a stream of moving matter. 



The rays are found to possess an unusual power of penetrating 

 matter impervious to light. They will even traverse a considerable 

 thickness of aluminum. A comparison of the absorbing powers of 

 different materials for cathode rays shows absorption to be roughly pro- 

 portional to the density of the substance. 



There is a field of magnetic force about a beam of these rays and 

 this added to the transfer of electricity along the path gives to the 

 cathode stream the distinguishing marks of an electric current in a 

 wire or a procession of electrically charged bodies. If a magnet be 

 brought near the tube the cathode stream is deflected from its direct 

 course. This deflection by the magnet shows three things : first, cathode 

 rays are not of the nature of light rays, the path of which a magnet is 

 powerless to change. Second, the curved path which the stream follows 

 again shows the stream to possess inertia. Third, the side to which the 

 rays are deflected indicates a stream of negative electricity. 



Strongly electrified bodies brought near the tube also deflect the 

 rays. It is possible to determine the speed and the ratio of charge to 

 the mass of the cathode particle, by measurements of the curvature 

 of the path due to the combined magnetic and electrostatic deflections. 

 Speeds as high as one tenth the velocity of light or 100,000 times the 

 speed of a modern rifle bullet have thus been observed. The ratio of 

 charge to mass comes out nearly a thousand times that found for the 

 hydrogen atom by electrolysis. If the charge on the cathode particle 

 is no larger than that on the hydrogen atom, which was called an atom 

 of electricity, then the inertia or mass of these particles is only one 

 one-thousandth part of the mass of hydrogen atoms. 



The nature of cathode rays was thus determined, but at this stage it 

 was all important to catch a known number of these missiles and meas- 

 ure the electric charge each carried. As the estimated size of these 

 minute bodies is less than one ten-million-millionths of an inch, direct 

 counting would be both slow and difficult, yet by one of the most in- 

 genious experiments ever performed, Professor J. J. Thomson did 

 it, indirectly. 



