of gaging-element impedance which is connected in parallel with one arm of the bridge. Drive 

 voltage is applied to the bridge from a balanced source, which permits grounding (earthing) 

 of one of the bridge terminals which is also connected through a cable (RG-62U) to the ground 

 side of the gaging element, (see Figure 2). This is a necessary condition since the body of 

 water in which the gaging element is to be submerged is used as the ground reference potential 

 point for the entire system. Unbalance voltages arising at the opposite terminal of this bridge, 

 which is initially manually balanced for still-water conditions, are applied to the attenuator 

 (block 4) and thence to the voltage amplifier and demodulator circuits (blocks 5 and 7). The 

 null indicator (block 6) is employed in order to ascertain when bridge balance has been achiev- 

 ed. The output of the demodulator circuit is a d-c valtage which varies from to 2 volts in 

 either polarity as a linear function of water height. The output impedance at this point is 

 quite high and should not be loaded heavily in the interest of linearity. Since very little 

 power can be drawn from this circuit, a direct-coupled power amplifier (block 8) is designed 

 to match the particular recorder chosen (block 9). Any desired recorder could, of course, be 

 used provided that a suitable power-amplifier circuit is substituted. 



Copies of the final schematic wiring diagrams of the gage control unit and the power 

 supply-amplifier unit are included as Figure 4 and Figure 5, respectively. 



RECORDER 



The instrum.ent described in this report is designed particularly for use with the San- 

 born* Model 60 direct-writing electromagnetic recorder. (This model is a two-channel unit, 

 which is used in those measurements where waves are to be measured at two points simultan- 

 eously.) This recorder is equipped with heated stylii and records on heat-sensitive paper. 

 Frequency response is uniform to about 40 cps, which is far more than adequate for this ap- 

 plication. The galvanometers (pen motors) are rugged and reliable, and, except for some 

 slight mechanical hysterisis which amounts to approximately 0.25 mm on the record, the re- 

 corder is quite satisfactory. With the amplifier shown in the schematic diagram. Figure 5, 

 full-scale deflections of ±2 cm from the center can be obtained with good linearity. One 

 great advantage realized by use of this recorder is the rectilinear recording feature which 

 facilitates analysis of recorded wave forms. A wide range of paper speeds is made available 

 by changing gears in the chart drive m.echanism; in addition a quick shift ratio of 10 to 1 may 

 be accomplished by a small lever located on the control panel. 



♦Manufactured by the Sanborn Company, Cambridge 39, Mass. 



