Resistors - 5 000 ohm 



II Resistors - 1000 ohm 



II Resistors - 200 ohm 



To Null Instrument 



Figure 4 - Diagram of the Decade Voltage Divider 



a flashlight bulb would light as soon as contact was made between the probe and the model. 

 For this purpose, a fine wire, a few mils in diameter, is stretched along the contour of the 

 model just above the water surface, and the auxiliary circuit is triggered when the probe just 

 touches the wire. The auxiliary circuit is opened before the potential reading is taken. 

 Careful manipulation of the micrometer screws enables contact with the body to be made with- 

 out danger of bending the tip. 



An electronic voltmeter with a low range of . 005 volts together with a preamplifier 

 was used as a null instrument. This instrument proved to be more sensitive than a cathode- 

 ray oscillograph in obtaining a null reading. As it was impossible to eliminate all pickup 

 from the circuit, it was difficult to obtain an undistorted Lissajous figure with the high ampli- 

 fication needed. 



Although the double probe is not useful for probing in contact with a model, it is very 

 useful for exploring the velocity and pressure fields away from the model. In probing in con- 

 tact with the model, it would not be possible to use the technique of a fine wire along the con- 

 tour and there would be danger of bending the tips and changing the spacing between them. 

 However, the double-probe method has several important advantages in exploring the field 

 away from the body. It is more convenient since potential differences, which are proportional 

 to the velocities, are found directly without making a differentiation. For detecting small 



