THE CATHODE RAY OSCILLOGRAPH 



15 



I shall describe it only as I describe the reason for the various features. 



First we wanted a convenient tube to operate on the batteries of 

 an ordinary vacuum tube. The thermionic filament cathode was 

 therefore required. The anode is a metal tube placed a short distance 

 from the cathode and between them is a metal disc with a perfora- 

 tion through which the electrons pass to the anode. These electrodes 

 are represented respectively by the letters C, A and D in Fig. 20. 



The electrons flow from the cathode to the inside of the anode and 

 some of them pass through and form the electron beam. Now, for 





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Fig. 20 — Diagram of electron gun. 



reasons to be mentioned presently, there is some gas in the tube and 

 this puts two requirements on the structure of the electron gun. First, 

 the gas would permit considerable ionization in the tube if the elec- 

 trons were permitted to pass into it. Most of the current would go 

 around the disc and to the outside of the anode. The cathode and 

 anode are therefore enclosed in a small tube, having dimensions less 

 than the mean free path of electrons in the gas so that no appreciable 

 ionization can build up. There is some ionization, however, in the 

 space between the cathode and anode, and the positive ions produced 

 tend to bombard the filament. If the filament is directl}' exposed to 

 this bombardment the oxide coating is worn oft", as in a sand blast, 

 in a matter of two or three hours. The filament is therefore wound 

 in the shape of a helix which is mounted coaxially with the anode and 

 the perforation in the disc so as to be out of the direct path of the 

 ions. In this way the filament is made to last several hundred hours 

 of operation. 



This completes the internal parts of the electron gun. Externally 

 it has mounted on it two pairs of deflector plates that control the 

 direction of the electrons after they leave the gun. In order to pre- 

 vent any large difference of potential between the anode and deflector 

 plates, one plate of each pair is connected directly to the anode, and 

 only the other plate has the variable potential impressed on it. As 

 for size and separation, the plates are designed to give maximum sen- 

 sitivity for a given full deflection. The sensitivity is about 1 mm. 



