CATHODE-RAY OSCILLOGRAPH 245 



trated by the grand mal epileptic seizures seen in humans is shown by the 

 electrical activity building up to a climax in parallel with the muscular 

 spasms or clonic jerks. In general, overexcitation of the cortex seems 

 to be revealed by quick sequences of sharp spikes at relatively high 

 voltage. 



It is still uncertain how the differences of potential are generated by the 

 brain and what the variations indicate. The groundwork of empirical 

 observation and physiological analysis for the interpretation of the 

 electrical activity of the brain is still being laid. 



Cathode-Ray Oscillograph 



Adequate study of the action currents in the central nervous system, 

 or allied phenomena, can be carried out only with instruments of great 

 sensitivity and resolving power. An amplifier must be available which 

 can step up transient signals of the order of 1 or 2 microvolts and deliver 

 them with enough power to operate a cathode-ray oscillograph. 



This type of oscillograph uses a fine jet of electrons, emitted by an 

 electron gun (see " Electron Microscope "), as a pencil to plot graphs on 

 a fluorescent screen of one electrical quantity as a function of a second 

 electrical quantity. In order to accomplish this, advantage is taken of 

 the fact that a jet of high-speed electrons can be deflected by an electric 

 field through which it passes. If the beam is allowed to impinge on a 

 willemite (ZnSi0 4 ) fluorescent screen (see " Intensifying Screens "), 

 a small spot of vivid green fluorescent light can be observed where the 

 collisions of the electrons with the screen take place. 



The deflection of the beam by an electric field, as observed by the 

 motion of the fluorescent spot, serves as a measure of the strength of 

 that field. Since such an electron beam is practically without inertia, 

 a sudden change in the electric field strength or a reversal of polarity of 

 the field is instantly recorded as a change in the position of the fluorescent 

 spot. 



Figure VI-20 shows the modern pear-shaped type of oscillograph 

 vacuum tube with an electron gun at G emitting a fine pencil of high- 

 speed electrons focused on the fluorescent screen by the electric lenses 

 Ai and A 2 - The fluorescent material is placed on the inner surface of 

 the glass face A. In the neck of the tube are two pairs of parallel 

 " deflection plates," x and y. The electron beam passes between these 

 plates along the central axis of the tube, after which it impinges on the 

 fluorescent screen at A, where a small luminous spot may be observed. 

 If coordinate axes are inscribed on the outer face of the tube any dis- 

 placements of the spot can be described as movements referred to these 

 coordinate axes. 



