Figure 4. — Field current meter, with parts labelled (wherever possible) to correspond with text. 



register counter (B) is reset to 0, and 

 actual tests are begun. 



The 18-volt (two 9-volt) battery, 

 switched on by E, is the power supply fo 

 the transistors (J and K) Euid the photo- 

 electric cell (H). The transistors act 

 two alternately operating switching unit 

 When only background light falls 

 on the photoelectric cell, by 

 means of resistances Rj^ and R2, 

 the bias voltage is set just suf- 

 ficiently enough to permit unit K 

 to conduct. At the same time, 

 resistances R4 and R^ establish 

 bias on unit J which keeps it from 

 conducting. When light falls on 

 the photoelectric cell from the 

 lamp (L), by means of resistances 

 R4 and R3, the bias voltage on 

 unit J becomes less negative and 

 the bias voltage on unit K more 

 negative. This causes unit K to 

 cease conducting Eind unit J to 

 begin conducting. The relay (I) 

 in series with section J is then 

 energized, activating the register 

 counter circuit. When light from 

 the lamp no longer strikes the 

 photoelectric cell, the bias vol- 

 tage on unit J becomes more nega- 

 tive and the bias voltage on unit 

 K less negative. This allows the 

 relay to return to its normal open 

 position and the circuit to come 

 to equilibrium. As previously 



the stated, the counter register records only 

 when light falls on the photoelectric cell 

 which occurs when the propeller makes a 

 complete revolution. 



The meter has been calibrated by 

 as passing known volumes of water through the 

 s. instrument at various speeds for known 



@^^' 



® 



BATTERIES 



REGISTER COUNTER 



SWITCH 



POTENTIOMETER 



TRIGGER SWITCH FOR (§) 



PHOTOCLETRtC CELL 



RELAY 



-TRANSISTORS 



LAMP 



Figure 5. --Electrical diagram of the field current meter. 



