VOLTAGE SENSITIVITY OF CATHODE-RAY OSCILLOGRAPH 247 



dons remains the same but their phase-angle difference is increased until 

 a circular trace of the spot is observed, then this circular pattern shows 

 that the phase difference has become 90°. 



If the x plates are connected to a source whose potential varies so that 

 the fluorescent spot moves along the X axis with constant speed and 

 very rapidly returns, then the circuit that produces such an effect 

 is called a sweep circuit. If at the same time a 60-cycle alternating 

 potential is applied to the y plates, the vertical harmonic trace would be 

 swept from left to right at constant speed, and with proper timing of the 

 sweep circuit one will observe, because of the persistence of vision, a 

 stationary sine-wave pattern. 



For the study of a transient bioelectrical phenomenon connected to 

 the y deflection plates, a high-speed accurate linearly increasing poten- 

 tial is used as a sweep circuit to trace the varying pattern as a function 

 of time. If the electrical activity is a mo no phasic action potential of a 

 nerve, a curve similar to that of Fig. VI-8 will be developed. 



The cathode-ray oscillograph is not a machine for performing any 

 desired operation upon an electrical signal impressed upon the deflection 

 plates of the instrument; it only traces on the fluorescent screen a pat- 

 tern which must be interpreted. In interpreting this pattern it should 

 be remembered that the unknown signal is always plotted as a function 

 of some signal whose characteristics are known. 



Voltage Sensitivity of the Cathode-Ray Oscillograph 



A difference of potential connected across either pair of deflection 

 plates of the oscillograph produces an electric field of strength E equal to 

 the difference of potential divided by the distance between them. An 

 electron of velocity v coming from a field-free space, on entering such a 

 field at right angles to the lines of force, is deflected along a parabolic path 

 (Fig. VI-21) as long as it remains in the electric field. After it clears 

 the lower edge of the plate, it reenters a field-free space and moves in a 

 straight line until it strikes the fluorescent screen. As a result, the beam 

 is depressed through an angle 6. The resulting vertical deflection y, 

 when the small distorting end effects of the field are neglected, is 



y = iat 2 



In its horizontal flight the electron travels the length of the plates x at 

 right angles to the field such that 



x = v x t 



