the counter trigger switch (G). The 

 potentiometer (F) should then be adjusted 

 so that impulses are being received evenly 

 on the counter. The register counter (B) 

 is reset to 0, and the actual tests are 

 begun. 



The power supply of the unit operates 

 on a 110-volt source, switched on by E and 

 provides a voltage of approximately 190 for 

 the plates of the 12AU7 tube (C) and for 

 the photoelectric cell (H) (fig. 3). The 

 12AU7 tube is a dual triode tube which acts 

 as two independent switching units (J and 

 K). When only background light falls on 

 the photoelectric cell, by means of resist- 

 ances Ri and R2, a bias voltage is set just 

 sufficient to permit unit K to conduct. 

 The resistance R. establishes bias on the 

 grid of section J which keeps this section 

 from conducting. When light falls on the 

 photoelectric cell from the Icimp (L), by 

 means of resistance R3, the bias voltage on 

 unit J becomes less negative, and that on 

 unit K more negative theui previously. This 

 causes unit K to cease conducting Jind unit 

 J to begin conducting. The relay (I) in 

 series with section J is then energized, 

 activating the register counter circuit. 

 Every time the openings in the propeller 



® 



Ri<l20K 



® 



® TRANSFORMER 



© REGISTER COUNTER 



© TUBE 



@ CONDENSER 



(D SWITCH 



© PHOTOMETER 



© TRIGGER SWITCH FOR (g 



® PHOTOELECTRIC CELL 



© RELAY 



S^SWITCHING UNITS 



© LAMP 



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assembly (fig. 2) are in direct line with 

 the openings of the lamp and photoelectric 

 cell units, light can fall on the photo- 

 electric cell and an impulse is received 

 on the register counter. This impulse is 

 recorded on the register and represents a 

 complete revolution of the propeller. 

 When the holer are no longer lined up, the 

 bias voltage on unit J becomes more nega- 

 tive while the bias voltage on unit K 

 becomes less negative. This allows the 

 relay to return to its normal open position 

 and the circuit to come to equilibrium. 

 The counter register records only when 

 light falls on the photoelectric cell, and 

 this occurs when the propeller makes a 

 complete revolution. 



DESCRIPTION AND OPERATION OF THE METER 

 AS DESIGNED FOR FIELD USE 



The field meter, like the laboratory 

 meter, is made of three main parts: chas- 

 sis (1) , rod assembly (2), and gun (3) 

 (fig. 4). As with the laboratory meter 

 illustrations (figs. 1, 2, and 3), wherever 

 possible, pcirts in figure 4 have been 

 labelled to correspond with the electricail 

 diagram (fig. 5), 



The chassis (12 inches 

 long by 8 inches wide by 3 

 inches high) is constructed of 

 steel (1/32 inch thick). It 

 houses trtinsistors (2N94A), 

 potentiometer, two 6-volt Jtnd 

 two 9-volt batteries, register 

 counter, relay, and main 

 switch. 



® 



R3<500K 



Figure 3. --Electrical diagram of the laboratory current meter. 



The rod and gun assem- 

 blies are identical with those 

 used in the laboratory appara- 

 tus (figs. 1 and 2). 



Operational procedure 

 in the field is identical with 

 laboratory procedure. The rod 

 assembly is submerged in the 

 water to the desired depth 

 (limited by the length of the 

 rod) and the test begun by 

 closing the switch E and re- 

 leasing the counter trigger 

 switch (G). The potentiometer 

 (F) is then adjusted so that 

 impulses are being received 

 evenly on the counter. The 



