250 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



Current from the 620-volt d-c source flows through R, charging the 

 condenser C. The charge on this condenser rises exponentially with 

 time, as illustrated by the broken saturation curve in Fig. VI-22. The 

 gas discharge tube does not allow this voltage to attain its maximum 



value V m , because the gas in 

 the tube ionizes at 120 volts. 

 At the instant the voltage has 

 reached this value (P on the 

 curve) the tube flashes, dis- 

 charging the condenser in the 

 very short time At. During 

 this discharge time the poten- 

 tial of the tube falls from P 

 until it reaches a lower critical 

 value V c at , where ioniza- 

 tion ceases. At this potential 

 the tube again becomes a non- 

 conductor, and the condenser- 

 charging cycle repeats itself, 

 the potential rising from 0' 

 to P' ', etc. Thus the " saw- 

 toothed" sweep-plate-voltage 

 graph is generated. 



If this generator is con- 

 nected to the deflection plates 

 of the oscillograph, the charg- 

 ing voltage sweeps the electron 

 beam across the screen while the discharge cycle returns it to its starting 

 point. The frequency with which this action occurs for a given flash- 

 voltage of a gas discharge tube is determined by the time constant RC. 

 Thus, either R or C, or both, may be increased to decrease the frequency 

 of repetition rate of the fluorescent spot. In commercial forms of oscil- 

 lographs this repetition rate can be varied from 2 to 50,000 sweeps per 

 second. 



If we examine the exponential rise in voltage in the interval from O 

 to P or O' to P' , it will be noticed that it is not quite linear so that the 

 speed of the moving fluorescent spot is not quite constant. In order to 

 obtain a more uniform speed of deflection a more linear charging interval 

 of the curve must be used. This may be found nearer the origin. A 

 gas discharge tube ionizing at a lower voltage moves the point P down 

 the curve. The logarithmic curve being steeper and more linear near 



Fig. VI-22. Broken line shows the expo- 

 nential voltage rise as the condenser C charges 

 through the resistance R. Units of time 

 depend on the product RC. With increase 

 in RC the repetition rate diminshes. P, P' 

 are the flash voltages of the tube G, while At 

 is its deionization time. Insert is a simplified 

 schematic diagram showing use of a gas-filled 

 glow tube as a relaxation oscillator. If an 

 885 thyratron is used, the point P is attained 

 at about 160 volts, and the maintaining volt- 

 age is about 16 volts. 



