signal of this pulse generator had a reversible polarity and a 10-cps to 

 1-mcps continuously adjustable repetition rate vith a pulse width range 

 from 0.05 to 10.0 ms. The sharply rising and dropping voltage pulse, 

 attained with this instrument, was found to give distinct edges to the 

 bubble rows. The characteristics of the output of this unit, such as pulse 

 width, period, and amplitude, were accurately monitored by a Tektronix 

 EM-503 Oscilloscope (see Figure 8). 



For clear and concise bubble rows at higher flow velocities, the out- 

 put of this pulse generator was amplified by the circuit shown in Figure 9- 

 Through this amplifier, the voltage transmitted to the wetted wire could 

 be increased to more than kOGV . 



LIGHTING 



Ordinary floodlights were found too bullsy to mount properly in the 

 hatch of the water tunnel, and they produced insufficient light . Accordingly, 

 the existing tunnel flash tubes (Edgerton, Germeshauser, and Grier FX-22) 

 were installed in the test section of the tunnel in their 28- inch glass tube 

 mounts in the hatch of the water tunnel (see Reference j) , They improved 

 the situation somewhat but could not be focused properly upon the bubble 

 rows. These lights were used for stroboscopic lighting and gave 5000-^, 

 kQOO-Y flashes. 



In order to attain increased clarity between the white bubble rows 

 and the black background, two 100-w Sylvania "sun guns" were mounted 

 beneath the hatch cover and above the water surface . Figure 10 shows two 

 "sun guns" mounted in the drained test sections. These very compact and 

 extremely bright lights proved very satisfactory for flow velocities as 

 high as '^ or 6 ft/sec . Not only can these particular lights withstand water 

 being splashed on them, but they can also operate continuously when com- 

 pletely submerged; the submerging advantage obviously eliminates the water- 

 surface reflection effect . This submerging characteristic is a distinct 

 convenience because the proper illumination of the bubble rows requires that 



15 



