. Elementary and dental radiography . f one cathode stream w^hile the other is beingused for X-ray production. Another scheme is to move one cathode backso far that the cathode stream focuses before reaching the back of thetarget. (Fig. 57.) The high-frequency tube may be used to advantage on an inductioncoil which is generating a great deal of inverse current. One may wonder why a valve (Fig. 51) or a rectifier (Fig. 27)could not be used to cut out the flow of current in one direction, and atube like the one in Fig. 44, for example, used on a high-frequencycoil. Neither the valve nor the rect
. Elementary and dental radiography . f one cathode stream w^hile the other is beingused for X-ray production. Another scheme is to move one cathode backso far that the cathode stream focuses before reaching the back of thetarget. (Fig. 57.) The high-frequency tube may be used to advantage on an inductioncoil which is generating a great deal of inverse current. One may wonder why a valve (Fig. 51) or a rectifier (Fig. 27)could not be used to cut out the flow of current in one direction, and atube like the one in Fig. 44, for example, used on a high-frequencycoil. Neither the valve nor the rectifier is capable of cutting out one X-RAY TUBES AND THE X-RAYS 6i direction of flow of a current of such high potential (voltage). Thevalve is able to cut out the inverse current of an induction coil becauseit (the inverse current) is comparatively weak; and the rectifier can cutout one wave of the commercial , because the voltage is only a littleover a hundred. The voltage of the high-frequency coil is perhaps ahundred Fig. 02. Table, with a lead glass protection sliield and compression diaphragm. To avoid straining a tube, it should not be again used, after hav-ing been used till hot, until it has cooled thoroughly. Overheating thetube will destroy the gases in it, and so raise the vacuum to such a degreeas to make it impossible to force a sut^cient milliamperage through it toproduce a sufficient number of X-rays. Sending a very strong currentthrough a tube of l(nv vacuum will also destroy the gases of the tul)e andspoil—strain—it. Fig. 58 is a tube rack for holding the tul)e when not in use. It is obvious that there must be some kind of a device for holdingthe tube when in use. This may be either a plain tube stand (Fig. 59),or a tube stand with a lead glass, protection shield and a compression 62 ELEMENTARY RADIOGRAPHY
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