THE CATHODE RAY OSCILLOGRAPH 



One of the more dramatic recent applications has been the investiga- 

 tion of lightning, probably the most important work on lightning 

 since its electrical nature was discovered by Benjamin Franklin one 

 hundred eighty years ago. 



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Fig. 3 — Tube with magnetic deflection. 



Let us examine the operation of the tube more closely. The speed 

 of the electrons as they emerge from the aperture in the anode can be 

 determined from the energy equation, 



\mv'^ = eV, 



the energy of motion equaling the total work done on the electron by 

 the field between cathode and anode. V is the potential between 

 cathode and anode, e the electric charge constituting the electron, m 

 its mass and v its speed. Solving for the speed we have the relation 

 between the speed and the driving voltage. 



4 



2-V. 

 ni 



The value of e/m is known to be 1.77 X 10^ e.m.u., the volt is 10« 

 e.m.u. and the velocity of the electrons is thus 



V = 5.95 X lO^VFcm./sec. 



Tf the driving potential is 300 volts, then the speed of the electrons is 

 given as roughly 1 X 10^ cm. /sec. or 6000 miles per second. For a 

 tube 20 cm. long an electron travels from the deflector plates to the 

 screen in 20/109 = 1/50,000,000 sec. If the applied voltage is 30,000 

 volts the speed is very nearly ten times as great as with 300 volts; it 

 is }4 the velocity of light. A change of direction of the ray induced 

 at the deflector plates is therefore transmitted to the end of the ray 



