. Dental and oral radiography : a text book for students and practitioners of dentistry . r arm. Before being finally sealed, the tube is pumpedto a high degree of vacuum (about 1/100,000 partof an atmosphere), only enough air being left init to afford a path for the -passage of the electriccurrent. Three general types of tubes are made for radi-ographic work, all of which embody the same gen-eral principles but vary according to the type ofthe machine upon which they are to be used. They are designated as follows: 1. The coil tube. 2. The transformer tube. 3. The high frequency tube. Coil tub
. Dental and oral radiography : a text book for students and practitioners of dentistry . r arm. Before being finally sealed, the tube is pumpedto a high degree of vacuum (about 1/100,000 partof an atmosphere), only enough air being left init to afford a path for the -passage of the electriccurrent. Three general types of tubes are made for radi-ographic work, all of which embody the same gen-eral principles but vary according to the type ofthe machine upon which they are to be used. They are designated as follows: 1. The coil tube. 2. The transformer tube. 3. The high frequency tube. Coil tubes and transformer tubes are similarin construction but not in vacuum (see Fig. 26).Coil tubes are exhausted to a much higher degree REQUISITES OF DENTAL X-RAY LABORATORY 69 of vacuum in order to lessen the tendency for in-verse current, and give a high degree of penetra-tion. The transformer tube is made compara-tively low in vacuum as the current from thetransformer is entirely free from inverse, andof such high voltage that the high vacuum isneither necessary nor advisable. P &. Fig. 28. The high frequency tube differs slightly in con-struction from the coil and transformer tubes ow-ing to the fact that the high frequency current isnot uni-directional. Therefore, a means must beresorted to for cutting away or disposing of onewave of the alternating current. This is accom-plished as shown in Fig. 27 by placing the anode 70 DENTAL AXD ORAL RADIOGRAPHY or target in the position occupied in the coil tubeby the assistant anode except that it extendsdown to the center of the tube. Then by havingwhat really amounts to two cathode terminals,only one of which is focused against the face ofthe anode, and the other into a funnel in the backof the target, almost the same effect is producedas results from an uni-directional current. Connecting the Tube to the X-Ray Machine In connecting up the tube to a coil or trans-former (Fig. 28), the anode terminal (A) is con-nected by means of a wi
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