. Elementary and dental radiography / by Howard Riley Raper . right, the negative, side of Fig. 49. The second means of eliminating inverse is by the use of thepatent spark gap. (Fig. 50.) The current passes easily from point todisc, but reluctantly from disc to point. Thus this series spark gap may beused to cut out inverse current from a tube, with the points toward thepositive terminal tape and the points away from the negative terminaltape. Fig. 50 shows the point toward the positive terminal tape. Theyshould be turned in the opposite direction at the negative terminal—awayfrom the tape. 5
. Elementary and dental radiography / by Howard Riley Raper . right, the negative, side of Fig. 49. The second means of eliminating inverse is by the use of thepatent spark gap. (Fig. 50.) The current passes easily from point todisc, but reluctantly from disc to point. Thus this series spark gap may beused to cut out inverse current from a tube, with the points toward thepositive terminal tape and the points away from the negative terminaltape. Fig. 50 shows the point toward the positive terminal tape. Theyshould be turned in the opposite direction at the negative terminal—awayfrom the tape. 54 ELEMENTARY RADIOGRAPHY The third means of cutting the inverse current out of an X-ray tubeis by means of a valve tube. (Fig. 51.) The valve, or Villard tube, is atube of low, or, as it is often called, Geissler vacuum—1/1,000 to 3/1,000of an atmosphere—with a disc electrode and a spiral electrode, both madeof aluminum. The exact reason for its action is not known, but the elec-tric current cannot travel through it well except in one direction—from. Fig. 52.
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