. Dental and oral radiography; a textbook for students and practitioners of dentistry . finite lineof demarcation. The hemisphere in front of the targetwhich is the active hemisphere, is evident by a fluores-cence deep apple green in color, while the other hemi-sphere should be evident by a 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 tubeto the x-ray machine as shown in Fig. 58. See that thethird terminal {S) is moved well away from the nega-tive terminal {S), or better still, disconn
. Dental and oral radiography; a textbook for students and practitioners of dentistry . finite lineof demarcation. The hemisphere in front of the targetwhich is the active hemisphere, is evident by a fluores-cence deep apple green in color, while the other hemi-sphere should be evident by a 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 tubeto the x-ray machine as shown in Fig. 58. 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- DENTAL AND ORAL RADIOGRAPIIY TECTINIC 109 ward each other until they are ahout three inches apart,and start the current. Unless the tube is loiu, the cur-rent will jump the spark gap instead of passing throughthe tube. If the tube resists the current and causes itto jump the spark gap, it is said to have backed upthree inches of spark. Thus a low tube will back up. two 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 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 will 110 DENTAL AND ORAL RADIOGRAPHY show a reading of 15 to 18, while with a medium tubethe reading will be from 10 to 12, and with a high tubethe milliamperemeter will register 5 or less. Relative Merits of Low, Medium, and High Tubes A low tube under average current conditions gives aclear sharp hemisphere of pal
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