478 



Dr, D. A. Kevs on a Piezoelectric 



turned on. On depressing the firing key, the cathode ray 

 spot traces a vertical line on the plate. It has time to 

 execute only three or four oscillations, when the further 

 depression of the key detonates the explosive mixture. The 

 key is now released and the cathode rays cease. The plate 

 is then lowered further exposing a fresh portion, and two 

 calibration lines are taken as follows : — The key is depressed 

 for an instant, producing a straight line on the plate. A 

 known potential difference is now applied between the plates 

 M and N and the key again depressed for an instant. This 

 produces another line displaced horizontally from the first, 

 the amount of this deflexion for a given potential difference 

 between M and N depending upon the cathode potential T. 

 This displacement is used as a check on the reading of the 

 electrostatic voltmeter. 



III. Calibration of the Apparatus. 



In order to obtain quantitative results, two sets of calibra- 

 tion measurements are required. In the first place, the 

 deflexion of the cathode ray beam will vary with the cathode 

 potential, since the velocity of the electrons depends upon 

 this potential, which is altered at every exposure until the 

 fine spots are obtained *. A constant potential difference of 

 17"5 volts was applied to the condenser plates and a series of 

 deflexions taken for various cathode potentials. The curve 

 shown in figure 3 is the resulting graph by means of which 

 displacements taken with any particular cathode potential 



Fig. 3. 



V80 



•-•60 

 •o 



°o-40 



I 

 |.20 



Q n 



2500 3000 4000 5000 6000 7000 



Cathode Potentiat in Vo/ts. 



Cathode Potential — Displacement Calibration Curve. 

 Displacement for 17 - 5 volts. 



may be reduced to those in terms of any other which may be 

 chosen as a standard. 



* See p. 477. 



