. Elementary and dental radiography . posts of the terminal tapes. (Fig. 49.) Series spark gaps (Fig. 50) may be used between the coil andterminal tape to cut out the inverse. The current passes easily frompoint to disc, but reluctantly from disc to point. Thus the series sparkgap may be used to cut out inverse current from a tube, with the pointstoward the positive terminal taj^e and the ])oints a\va\ from the negativetcnninal tape. li,q. 50 sliows tlic ]M)ints toward the jiositive terminaltape. They should be turned in the opposite direction at the negativeterminal—away from the tape. 54 ELE
. Elementary and dental radiography . posts of the terminal tapes. (Fig. 49.) Series spark gaps (Fig. 50) may be used between the coil andterminal tape to cut out the inverse. The current passes easily frompoint to disc, but reluctantly from disc to point. Thus the series sparkgap may be used to cut out inverse current from a tube, with the pointstoward the positive terminal taj^e and the ])oints a\va\ from the negativetcnninal tape. li,q. 50 sliows tlic ]M)ints toward the jiositive terminaltape. They should be turned in the opposite direction at the negativeterminal—away from the tape. 54 ELEMENTARY RADIOGRAPHY The third, last, and the most efficient means of cutting theinverse current out of an X-ray tube is by means of a valve tube. () The valve, or Villard tube, is a tube of low, or, as it is often called,Geissler vacuum—1/1,000 to 3/1,000 of an atmosphere—with a discelectrode and a spiral electrode, both made of aluminum. The exactreason for its action is not known, but the electric current cannot travel.
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