File:Dental and oral radiography - a text book for students and practitioners of dentistry (1916) (14570573970).jpg

Identifier: dentaloral00mcco (find matches)
Title: Dental and oral radiography : a text book for students and practitioners of dentistry
Year: 1916 (1910s)
Authors: McCoy, James David
Subjects: Radiography, Dental X-rays Teeth Mouth
Publisher: St. Louis : Mosby
Contributing Library: West Virginia University Libraries
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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esignated as high, medium or low, a high tubebeing one in which the vacuum is well nigh com-plete; in a medium tube the vacuum is lesscomplete, while a low tube is one in which thevacuum is far from complete. For dental radiography a fairly high tube isindicated, as with such a tube x-rays may be pro-duced having a degree of penetration sufficient topass through the oral structures and produce thedesired effect upon the emulsion of the plate orfilm. When a current of high voltage is passedthrough such a tube, it should light up in a char-acteristic manner forming two hemispheres which DENTAL AND ORAL RADIOGRAPHY TECHNIC 111 have a definite line of demarcation. The hemi-sphere in front of the target which is the activehemisphere, is evident by a florescence deepapple green in color, while the other hemisphereshould be evident by a lack of greenish light. To Determine the Vacuum of a Tube The comparative degree of vacuum of a tubecan be determined in the following manner: Con- PC*=f>
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Fig. 50. nect the tube to the x-ray machine as shown inFig. 50. See that the third terminal (S) ismoved well away from the negative terminal (#),or better still, disconnect the wire running to theregulating chamber (R). Now, move the slidingrods (B and D) of the secondary spark gap, to- 112 DENTAL AND ORAL RADIOGRAPHY ward each other until they are about three inchesapart, and start the current. Unless the tube islow, the current will jump the spark gap insteadof passing through the tube. If the tube resiststhe current and causes it to jump the spark gap,it is said to have backed up three inches ofspark. Thus a low tube will back up two orthree inches of spark, a medium tube five orsix inches, while a high tube will back up six oreight inches. In fact, the vacuum of a tube maybe so great that only the most powerful x-raymachines will operate it. Such a tube, however,is not useful for dental radiography. The vacuum of a tube may also be determinedby the use of an instrument known as a

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