232 



Mr. J. A. McClelland. 



the tube in which the cathode rajs were produced, and a second tribe 

 which could be exhausted independently of the first, and in this tube 

 he observed the deflection produced by a magnet on the rays after 

 they had passed through the aluminium, or, on our way of looking 

 at it, on the secondary rays produced in this tube by the rapid 

 approach of the negatively charged particles, in rushes, up to the 

 aluminium window. Lenard found that if the conditions in the dis- 

 charge tube remained the same, he could vary the pressure in his 

 observing tube within wide limits without producing any change in 

 the amount of deflection of the rays. This at first sight appears to 

 show that the Lenard rays are different in character from cathode 

 rays, because we know that when the pressure in a discharge tube is 

 diminished the cathode rays are less deflected by magnetic force. 



But when we diminish the pressure in a discharge tube wc at the 

 same time increase the difference of potential required to make the 

 discharge pass, and if we regard the negative rays as negatively 

 charged particles shot off from the negative electrode, this increase of 

 potential difference will increase the momentum given to these 

 particles, and consequently they will be less deflected. We may thus 

 regard the change of pressure as changing the deflection not directly, 

 but because it changes the difference of potential between the 

 electrodes. 



We cannot test this view, experimentally, inasmuch as we cannot 

 vary the pressure and keep the difference of potential constant, but 

 we can keep the pressure constant and vary the difference of poten- 

 tial to some extent. If a spark gap is placed in series with the 

 discharge tube, by increasing the length of the spark we may make 

 the difference of potential between the terminals of the tube greater 

 than that required to make the discharge pass, and it is found that 

 forcing up the potential difference in this way does diminish the 

 deflection of the cathode rays. 



In an experiment a cylindrical tube of 1"5 cm. radius was used with 

 the cathode at one end. A central beam of cathode rays was de- 

 flected by a magnet on to the side of the tube so as to form a w^ell 

 defined patch of phosphorescence. When the spark gap was closed, 

 this patch was at a distance of 38 mm. from the cathode. Increasing 

 the length of the spark gap up to 10 mm. was sufficient to make the 

 patch travel away to 44 mm. from the cathode. 



This shows that without any change in pressure, an increase in 

 potential difference is sufficient to diminish the deflection. 



Now in Lenard's arrangement a change of pressure in the observing 

 •tube has no effect on the forces producing the rays in that tube, 

 if we regard them as produced by the rapid pulses of charged 

 particles up to the aluminium screen. The impulse starting the 

 rays is not varied, and consequently we would expect no change in 



