242 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



and 100 //A/lumen, the radiant sensitivity is between 0.002 and 0.1 

 /xA[ju\V for illumination at peak spectral sensitivity. Vacuum cells 

 are stable and do not change their characteristic over long periods of 

 time, particularly when operated at low plate voltages and pro- 

 tected from excessive light. 



Constructions of common photoelectric cells are shown in Figs. 

 (5-l)8a to d. Low values of leakage current (order of magnitude 10~ 10 







50 100 150 



Anode voltage, volts 



200 



Fig. (5-1)9. Voltage-current characteristic of a vacuum 

 photoelectric cell {from RCA Tube Manual, Tube 6570; 

 by permission). 



amp) are obtained with types in which the connection to one elec- 

 trode is made on top of the bulb. The dark current of tubes in which 

 both connections are made through the tube socket is of the order of 

 10 -8 to 5 x 10~ 9 amp at room temperature and a plate voltage of 

 about 200 volts. 



Typical voltage-current characteristics are shown in Fig. (5-1)9. 

 After an initial rise of the plate current with increased plate voltage 

 (transition from space-charge-limited to emission-limited operation) 

 the current remains almost constant with further increase of the 

 plate voltage (saturation region). The small remaining increase of 

 current in the saturation region is brought about by electrons which, 

 because of their initial velocity, bypass the anode, circle around it, 

 and return to the cathode region. The effect is reduced at higher 

 plate voltages. 



The current in the saturation region is essentially proportional 

 to the incident light flux. A small deviation from linearity (usually 

 of the order of 1 per cent) is due to field deformation in the tube. 



