56 DETERMINATION OF SPEED [CH. v. 



the magnet is too strong the screen S will become 

 dark, because the oscillating rays will be too much 

 deflected, so it is possible to tell pretty sharply the 

 phase of the oscillating rays in the right-hand portion 

 of the tube by watching the screen S and adjusting 

 the strength of the magnet. 



So much for the right-hand end of the tube. Now 

 proceed to the left-hand end, or rather to the 

 movable portions depicted to the left of figs. 6 and 7. 

 We have there also an oscillating current, M'N', in 

 precisely the same phase of standing oscillation as 

 its other branch MN ; consequently, if the rays take 

 no time to travel down the tube, those which get 

 through the disk B' will be deflected as before ; 

 but if the time taken to travel down the tube 

 corresponds with a quarter of the extremely rapid 

 oscillation-period of the condensers HK, then the 

 rays deflected a maximum amount by the first branch 

 of the circuit will not be deflected at all by the 

 second, and therefore will reach the middle of the 

 screen. 



By either altering the frequency of oscillation, or 

 adjusting the distance apart of the parts represented 

 by BB', it is possible to cause the deflexion on the 

 left-hand side to be either in the same phase, or in 

 opposite phase, with the first half; or to be a zero, of 

 the first, second, or third order. 



In this way, though obviously the experiment is a 

 difficult one, Wiechert was able to make measure- 

 ments of the speed of the cathode rays produced 

 under given circumstances. 



Unfortunately this speed is not a fixed and definite 

 constant, like the velocity of light : it does not indeed 

 depend upon the nature of the gas in the tube, nor 

 on the substance of the electrodes, but it does vary 



