52 DETERMINATION OF SPEED [CH. v. 



4 



simply the ratio of the strengths of the electric and 

 magnetic fields (both expressed in EM units, that 

 is with M=!) which can produce equal effects 

 upon the flying particles, and which therefore, if 

 applied in opposition, are just able to neutralise 

 each other. 



Note on Dimensions. 



To verify that the " dimensions " of the last given 

 equation are correct, we can remember that an electric 



F TT 



field is of dimension = -= ., _., where F is force, 



e I v (/of ) 



and I is length ; and that a magnetic field is of 



TT T? 



dimension = ^ ., _ x ; so the ratio of an electric 



m TjfjdF) 



to a magnetic field is */(V/K) ; wherefore -th of that 



1 M 



ratio is r/ r = a velocity. 



vV) 



For practical purposes it may be convenient to 

 write the equation as a proportion sum thus : 



the velocity of the particle 

 the velocity of light 



the electric field in electrostatic units 

 the magnetic field in electromagnetic units ' 



it being understood that the fields are adjusted till 

 their effects are equal. 



Effect on Lenard Rays. 



Another mode of demonstration was employed by 

 Professor Lenard, and served to identify his rays with 

 escaped cathode rays. 



