. Elementary and dental radiography / by Howard Riley Raper . eeps the end of the point resting against the porcelain and sothe amount of the point exposed remains the same. (Fig. 79.) The parallel spark gap may be manipulated to determine the numberof inches of spark the tube will back up and so determine the degree ofvacuum. (Pages 42 and 43.) As the operator becomes familiar with his 88 DENTAL RADIOGRAPHY tubes and machine he will manipulate this gap less. It is the practice ofsome men to set this gap at about 7 inches and leave it there, resorting tothe regulation of the vacuum of the tube
. Elementary and dental radiography / by Howard Riley Raper . eeps the end of the point resting against the porcelain and sothe amount of the point exposed remains the same. (Fig. 79.) The parallel spark gap may be manipulated to determine the numberof inches of spark the tube will back up and so determine the degree ofvacuum. (Pages 42 and 43.) As the operator becomes familiar with his 88 DENTAL RADIOGRAPHY tubes and machine he will manipulate this gap less. It is the practice ofsome men to set this gap at about 7 inches and leave it there, resorting tothe regulation of the vacuum of the tube only when sparking occurs at theparallel spark gap. The parallel spark gap control is found on all induc-tion coils. The vacuum reducing lever is the lever by means of which thetube-regulating spark gap (Pages 46 and 47) is made wide or the tube will light properly, and no sparking occurs at a 6 to 8-inchparallel spark gap, the tube requires no vacuum regulation. If sparkingoccurs across 6 to 8 inches of parallel spark, reduce the distance of the. Fig. 79. Self-adjusting electrolytic interrupter. tube-regulating spark gap to an inch or two and, with the rheostat on alow button, turn on the current for 2 or 3 seconds. This may be repeatedif necessary and will lower the vacuum of the tube. As tubes get old,the vacuum gets high, and they become cranky. To handle an oldtube it is often necessary to open the parallel spark gap as wide as pos-sible, and set the tube-regulating spark—5 inches for a slightly cranky tube, as short as 2 inches for a very high cranky tube—allowing the MAKING DENIAL RADIOGRAPHS 89 tube-regulating- spark gap to remain set all the time the current is turnedon. The object of the operator should be to avoid regulation of thevacuum of the tube as much as possible. The vacuum reducing lever control is not found on all inductioncoils. When the machine does not afford this control, the width of thetube-regulating spark gap is controlled by means of
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