LOIV VOLTAGE CATHODE RAY OSCTELOGRAPfl 



145 



length of the anode and form the beam in the main part of the tube. 

 The deflector plates, p, are also mounted rigidly on this unit. In 

 order to avoid large differences of potential in the tube, one plate from 

 each pair is permanently connected to the anode, the variable poten- 

 tials being applied to the other plates. The complete unit is mounted 

 at the small end of the tube with the anode and deflector plates toward 

 the fluorescent screen. 



The Filament 



In some earl>' forms the filament was bent into a simple hair pin 

 loop which was placed close to the aperture in the shield. It was then 

 found that the positive ions striking the filament from the direction 

 of the anode soon destroyed the oxide coating and left the filament 

 inactive. This trouble was largely overcome by placing the filament 

 out of the direct path of the positive ions. The flat filament is now 

 shaped into a ring as shown in P'ig. 3, slightly larger in diameter than 



BRAUM TU&E FILAMEhT 



Fig. 3 



the aperture in the shield and is placed coaxial with the anode. The 

 momentum of the positive ions then carries them past the active 

 part of the filament and they strike where little damage can be done. 

 The length of service of the tube is still limited by the filament life, 

 but this has been increased by the above artifice so that the tube now 

 gives around 200 hours of actual operation. 



