Tuned Circuit 



Figure 3 - Wiring Diagram of the Electric 

 Circuit Used with the Single Probe 



The construction of the voltage di- 

 vider R is shown in Figure 4. The resistors 

 in each bank are noninductive precision re- 

 sistors, accurate within ± 0.25 percent. A 

 slide wire potentiometer is used in the last 

 stage for fine adjustment. Thus the poten- 

 tial at the probe may be read to six figures, 

 with an uncertainty in the fifth or sixth 

 place depending upon the stability of the rest 

 of the circuit. Although an accuracy of four 

 significant figures in the potential may be 

 sufficient for obtaining a pressure distribu- 

 tion about a large model with gradual changes 

 in curvature, it is necessary to obtain five 

 and sometimes six significant figures in the 

 potential about small bodies where the curva- 

 ture changes rapidly. 



The sensitivity and stability of the electrical circuit are improved by eliminating elec- 

 trical pickup and ground loops, by avoiding polarization of the electrolyte at the probe and 

 electrodes, and by eliminating phase shifts in the circuit to the null instrument. 



To eliminate ground loops and extraneous pickup, isolation transformers are inserted in 

 the input and output circuits and the leads from the probe are shielded. The insertion of a 

 tuned circuit across the leads to the null instrument excludes extraneous frequencies and 

 greatly improves the sensitivity of the null reading. 



Polarization of the electrolyte about the probe and the electrodes manifests itself as an 

 instability in the null reading. In those experiments where four-place accuracy in the poten- 

 tial is sufficient, polarization was not particularly troublesome even when a 60-cycle power 

 supply was used. If greater precision is needed in the potential readings, precautions have to 

 be taken to eliminate polarization. Polarization may be reduced by increasing the frequency 

 of the power supply and by coating the probe and electrodes with Aquadag. With the Aquadag 

 coatings, the null reading could be stabilized to give five significant figures in the potential 

 reading with a 400-cycle input and was further improved with a 1000-cycle power input. Polar- 

 ization was found to be less troublesome with the tank full of water than with a depth of only 

 a few inches. 



Little is gained in obtaining great precision in the potential reading unless the position 

 of the probe can be determined with like precision. For this reason, micrometer adjustments 

 for positioning the probe were installed. In probing in contact with a model, it is extremely im- 

 portant to bring the probe into contact with the model without bending the tip. An error of a 

 few hundredths of an inch in the position of the probe may result in an error of several percent 

 in a pressure distribution. To avoid bending the probe, an auxiliary circuit was set up in which 



