VISUAL INDICATORS 



The value of P.D.A. tubes lies mainly in securing an adequately bright 

 trace when using a triggered time base and very high sweep speeds. As 

 normally used, sweep speeds in electrophysiological work seldom exceed 

 6 in. in 100 jusec, and for this conventional tubes are generally adequate. 

 However, in using special techniques, such as multi-channel oscillography 

 employing the voltage-coincidence method — to be described later — higher 

 writing speeds are encountered and P.D.A. tubes are of great help. 



Magnetic shielding of electrostatic tubes — Electrostatic tubes generally 

 need to be shielded from the effects of stray magnetic fields from devices 



PDA+ 



(a) 



(b) 



Figure 32.21 



such as mains transformers. To this end, mumetal screens, which fit closely 

 round the outside of the glass envelope, may usually be obtained from the 

 tube manufacturers. The high permeability of mumetal is destroyed by cold 

 working, so these screens should be subjected to as little mechanical distor- 

 tion as possible when being fitted. In exceptionally bad cases one screen 

 may prove to be insufficient; when this happens it is better to use two thin 

 screens, concentrically arranged, than one thick one^. 



Magnetic cathode ray tubes 



Magnetic cathode ray tubes have not hitherto been much used in 

 scientific oscillography, probably because the ancillary circuitry is more 

 difficult to design. However, in-so-far as they are more satisfactory for 

 television and radar, a great deal of research and development has been 

 devoted to them and their associated components in the last 20 years, and 

 it seems to the author that the use of magnetic tubes may now be at least 

 considered for laboratory applications. Their advantages are: 



(1) A brighter and better focused spot. 



(2) The tubes can be much shorter for a given screen diameter. In the 

 construction of large demonstration oscilloscopes for lecture-theatre use 

 the equipment is not excessively bulky. 



(3) Because television serves a mass market, tubes with relatively enormous 

 screens are readily available, whereas a comparable electrostatic tube would 

 have to be specially made, at much greater expense. At present magnetic 

 tubes range from models having round screens of 5 in. diameter, to rect- 

 angular screens of 21 in. diagonal. It must be admitted that this argument 

 weighs only in considering demonstration oscilloscopes. 



(4) Because of the mass market for television, suitable pieces of ancillary 

 equipment such as focus magnets and deflector coil assemblies are cheaply 

 and readily available. 



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