THE CATHODE RAY OSCILLOGRAPH 



of a certain length L, then the relation of spacing to length of the 

 plates must be 



I ~ L' 



as can easily be seen from Fig. 4. When this condition is satisfied it 

 makes little difference in the sensitivity whether the plates are large and 

 far apart or small and close together. 



The magnetic sensitivity of the tube is more uncertain only because 

 the boundaries of the magnetic field are usually less well defined than 

 those of the electric field. One form of the derivation will be given 

 here. If the ray generated by the driving voltage V passes for a dis- 

 tance / (Fig. 5) through a region in which there is a transverse mag- 



Fig. 5 — Magnetic deflection. 



netic field of strength if, the path of the beam is an arc of a circle 

 whose radius is 



R = 



mv 



1 \lmV 



eH H 



When the ray leaves the magnetic region it proceeds in a straight 

 line to the screen, where the total deflection from the normal is D. 

 If the angular deflection 6 is not great then we have, very nearly, 



tan = -y = ^ = /H 



Li K 



2mV 



and hence 



D = ILH 



4 



ImV 



In practical units instead of electromagnetic the expression becomes 



D = .MLH/4V. 



