. Elementary and dental radiography / by Howard Riley Raper . inecommonly met with.) Ordinary X-ray tubes, gas tubes, as they are calledsince the advent of the new Coolidge tube, could not take a current of60 milliamperes for much longer than about 1 second continuously with-out injury to the tube from overheating. When the time of exposure is over 5 seconds it is best to make theexposure intermittently: turn the current on for 5 seconds then off for 5seconds, then on for 5 seconds and so on until the desired time of exposure 126 DENTAL RADIOGRAPHY is given. This avoids overheating the tube, a
. Elementary and dental radiography / by Howard Riley Raper . inecommonly met with.) Ordinary X-ray tubes, gas tubes, as they are calledsince the advent of the new Coolidge tube, could not take a current of60 milliamperes for much longer than about 1 second continuously with-out injury to the tube from overheating. When the time of exposure is over 5 seconds it is best to make theexposure intermittently: turn the current on for 5 seconds then off for 5seconds, then on for 5 seconds and so on until the desired time of exposure 126 DENTAL RADIOGRAPHY is given. This avoids overheating the tube, and so lowering of the vacuumby heating the regulating chamber. When the time of exposure is I or 2 seconds, or less, an automatictime-switch may be used to advantage. Fig. 116. X-ray tubes should not be used until they become so hot the hand cannot be placed on them. Heating takes place particularly in the region ofthe cathode. If used until hot a tube should be allowed to cool before itis used again. If much radiographic work is being done it is economy to. Fig. 11G. Automatic time switch or timer, by means of which seconds may be split. have at least three tubes. (The new Coolidge is an exception to this ruleas it is to almost all rules regarding X-ray tubes.) With the milliamperage, the condition of the tube, and the emulsionon the plate remaining the same, it requires an exposure about four timesas long for an extra-oral dental radiograph as for an intra-oral one. Thisincrease in the length of time necessary for the extra-oral dental radio-graph is clue to increased distance between the target and film, increasedthickness of tissue to be penetrated by the rays and increased distancebetween the object being radiographed and the plate. MAKING DENTAL RADIOGRAPHS 127 The exposure necessary for frontal sinus and antrum radiographs with the pose as in Figs. 111 and 114. is longer than for radiographs ofany other part of the body. To avoid straining tuhes for radiographs ofthis ki
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