. Dental and oral radiography : a textbook for students and practitioners of dentistry . uum is medium or fairly high it should lightup in a characteristic manner forming two hemisphereswhich have a definite line of demarcation. The hemi-sphere in front of the target which is the active hemi-sphere, is evident by a fluorescence deep apple green incolor, while the other hemisphere should be evident bya lack of greenish light. To Determine the Vacuum of a Tube The comparative degree of vacuum of a tube can bedetermined in the following manner: Connect the tube 112 DENTAL AND ORAL RADIOGRAPHY to
. Dental and oral radiography : a textbook for students and practitioners of dentistry . uum is medium or fairly high it should lightup in a characteristic manner forming two hemisphereswhich have a definite line of demarcation. The hemi-sphere in front of the target which is the active hemi-sphere, is evident by a fluorescence deep apple green incolor, while the other hemisphere should be evident bya lack of greenish light. To Determine the Vacuum of a Tube The comparative degree of vacuum of a tube can bedetermined in the following manner: Connect the tube 112 DENTAL AND ORAL RADIOGRAPHY to the x-ray machine as shown in Fig. 56. See that thethird, terminal (S) is moved well away from the nega-tive terminal (S), or better still, disconnect the wire run-ning to the regulation chamber (R). Now, move thesliding rods (B and D) of the secondary spark gap, to-ward each other until they are about three inches apart,and start the current. Unless the tube is low, the cur-rent will jump the spark gap instead of passing throughthe tube. If the tube resists the current and causes it. Fig. 56. to jump the spark gap, it is said to have backed upthree inches of spark. Thus a low tube will back uptwo or three inches of spark, a medium tube five orsix inches, while a high tube will back up six or eightinches. In fact, the vacuum of a tube may be so greatthat only the most powerful x-ray machines will operateit. Such a tube, however, is not useful for dental radiog-raphy. The vacuum of a tube may also be determined by theuse of an instrument known as a milliamperemeter. DENTAL AND OEAL RADIOGRAPHY TECTINIC 113 This instrument which is usually an accessory of eitherthe induction coil or transformer, is connected in circuitwith the tube, and measures the current passing throughthe tube. With a low tube the milliamperemeter willshow a reading of 15 to 18, while with a medium tubethe reading Avill be from 10 to 12, and with a high tubethe milliamperemeter will register 5 or less. Relati
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