. Radiography and radio-therapeutics . point is the positive poleauil the negative the plate. Fig. 56.—Connections of the X-ray tube to thecoil, showing the appearance when the polesare reversed. cathode looks as if it were evenly filled with green hght, the other halfof the tube behind the anti-cathode remaining dark, because the anti-cathodeacts as a screen. If wrongly connected there isan irregularly patchy fluorescenceof the walls of the tube, and raysappear at intervals which changeconsiderably according to theamount of current passing throughthe tubes. For the appearance of theX-ray tube
. Radiography and radio-therapeutics . point is the positive poleauil the negative the plate. Fig. 56.—Connections of the X-ray tube to thecoil, showing the appearance when the polesare reversed. cathode looks as if it were evenly filled with green hght, the other halfof the tube behind the anti-cathode remaining dark, because the anti-cathodeacts as a screen. If wrongly connected there isan irregularly patchy fluorescenceof the walls of the tube, and raysappear at intervals which changeconsiderably according to theamount of current passing throughthe tubes. For the appearance of theX-ray tube when correctly andincorrectly in operation, see thecoloured plate. The important thing to re-member is first of all to ascertainthe polarity of the coil. This isdone by testing with the spark-gap. Selection and Reg-ulationof X-Ray Tubes—Little need besaid on this point. As nearly all the tubes on the market are now good, the particular type selected dependsto a large extent upon the operator. When possible a number of good tubes. Fig, 57.—Connection of X-ray tube to the coil,showing coil and connections to tube. A valvetube is inserted on the negative pole. 72 RADIOGRAPHY should be kept in constant use. A tube which is in good condition andhas a sharp focus should be reserved for radiographic work ; for therapeuticsa tube with a diffused focus is better than a sharp one because it will lastlonger. This requires to be taken with reserve. Some workers prefer touse a sharp focus for therapeutic work. It may be that the sharp focusof the cathode stream upon the anti-cathode may generate a beam ofX-rays of particularly good therapeutic value. Unless for special purposes it is always better to purchase tubes of amedium vacuum, inclining towards the soft side. A tube of large diameter,7 or 8 inches, will continue to keep good vacuum longer than a smaller heavy currents, either in radiographic or therapeutic work, the largertube will in the end be found most economical.
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