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47933 



Figure 2a - Probing Along the Contour of a Wax 

 Model of a Half Body 



Figure 2b - Probe Mount Showing the Micrometer 

 Screws for x, y, and z Motions 



Figure 2 - Views of the Probe Mounted on the Cross Beams 



at the water surface. Since it is difficult to hold tolerances of a few thousands of an inch 

 with wax models it will be necessary to machine future models out of plastic material. 



The pantograph shown in Figure la is useful for plotting lines of constant electrical 

 potential in the vicinity of a body. In two-dimensional flow, streamlines are easily obtained 

 as lines of constant electrical potential if the body is a conductor and Analogy B is used. 

 Points may be located within +0.01 in. with the pantograph. 



If the velocity and pressure distributions about a body are required, greater precision 

 may be obtained by using the probe mounted on a carriage (Figures lb and 2). The carriage 

 is supported by cross beams which move longitudinally on rails parallel to the sides of the 

 tank. Longitudinal and transverse motions may be adjusted by micrometer screws. The height 

 of the probe may be varied by means of a coarse screw with a calibrated head. Hence, dis- 

 placements in all three directions may easily be determined within 0.001 in. or better. 



Two methods were investigated for finding pressure distributions, the double-probe 

 method first described by Taylor and Sharman^ and the single-probe method. Analogy A is. 

 used in both methods and the potentials or potential differences are obtained with potentio- 

 meter circuits and a null instrument. The single probe is used for probing in contact with the 

 body, and the double probe is used for obtaining the pressure field away from the model. 



The circuit used in exploring the pressure distribution with a single probe is shown in 

 Figure 3. A pair of copper electrodes is set up parallel to each other at opposite ends of the 

 tank and is fed with 110-volt alternating current. Tap water was found to be a satisfactory 

 electrolyte. The potential at the probe is obtained by adjusting the capacitance C and the 

 voltage divider R for a null reading of the null instrument. The ratio of the potential at the 

 probe to the potential across the tank is read from the voltage divider. 



