The clarity of the hubble lines is very sensitive to the cleanliness 

 of the cathode vire . Removal of debris which accumulates on the wire can 

 be achieved in several ways = For example, the wire may be removed from the 

 water and carefully etched with a 20-percent solution of nitric acid. How- 

 ever, debris accumulates so frequently that this method becomes cumbersome. 

 When convenient access to the cathode wire or its wire holder is available, 

 a delicate striking of the wire holder with an electrically insulated object 

 suffices to loosen accumulated debris . It is of considerable importance to 

 incorporate this accessibility feature into the design of any closed test 

 section in which the hydrogen-bubble technique is to be used. Another is to 

 strike the metal side of the water tunnel with a small hammer. 



The following method, which was discovered quite by accident, is fre- 

 quently more efficient and incomparably more convenient than any of the 

 others cited. In order to observe the bubble lines from an anode wire, the 

 pulse polarity was reversed on a wire which had been just operating for some 

 time as a cathode hydrogen-generating wire. When the polarity was, changed 

 back again, the resulting hydrogen-bubble lines were amazingly concise and 

 distinct . The technical details of such a wire-cleaning procedure were not 

 investigated but it was felt that it was due to an electrostatic repulsion 

 process . Therefore, it has been found very convenient to arrange the 

 energizing circuit so that one has such a means available to clean the wire 

 while it is in the moving flow . 



Another method which has been found effective is to energize the wire 

 before the fluid is in motion. The generated hydrogen rises immediately 

 and an up'ward flow is produced all about the vertical wire which sweeps 

 debris up and off the vertical wire. However, of all these methods, the 

 polarity reversal methos is the one most :frequently used. 



SOME PRELIMINAEY EXPERMEIWAL RESULTS 



The bodies about which the flow was visualized are shown in Figure 13 . 

 The foil shapes are TMB m.odified WACA 66 profiles. Chord-to-thickness 

 ratios are 10:1 and 5:lj both chord lengths are 6 in. The circular 



18 



