. Röntgen rays and electro-therapeutics : with chapters on radium and phototherapy . ed during the pro-cess of blowing. Oiten tubes are pro-vided with an auxiliary anode, whichis invariably made of aluminium. In order that the shadow-picturemay be sharply defined, the cathoderays must converge or be focused toa point, and to meet this requirement the aluminium cathode must pre-sent a concave surface, varying in diameter from ^ to 1 inch (. cm.).The anode (anti-cathode) is made of platinum, and may have a varyingdiameter, but it is usual to have its plane so adjusted as to form anangle of
. Röntgen rays and electro-therapeutics : with chapters on radium and phototherapy . ed during the pro-cess of blowing. Oiten tubes are pro-vided with an auxiliary anode, whichis invariably made of aluminium. In order that the shadow-picturemay be sharply defined, the cathoderays must converge or be focused toa point, and to meet this requirement the aluminium cathode must pre-sent a concave surface, varying in diameter from ^ to 1 inch (. cm.).The anode (anti-cathode) is made of platinum, and may have a varyingdiameter, but it is usual to have its plane so adjusted as to form anangle of 45 degrees to the cathode. Placed in this position the anodebehaves like a reflector, receiving and throwing off the rays emitted fromthe cathode. Platinum has a high fusing ]Doint, and it is superior inthis respect to all ordinary metals; its use as the target for the cathodebombardment is because of its infusibility. Very few other metals maybe used in its place. The most important of these substitutes is irid-iiim, which is another member of the platinum group and has a higher. Fig. 88.—Essential features of an X-ray tube. THE EONTGEN EAY APPAEATUS. 179 fusion point than platinum itself. Osmium, which is also an infusiblemetal of the same group, might be used. Alloys of these metals, having varying percentages, are also used. These infusible metals are to be backed up by ordinary metals suchas copper and iron, because the latter are less expensive and readily pro-vide a large radiating surface and thermal capacity for the dissipation ofthe heat produced at the focus-point. The cathode is made of aluminium, because this metal disintegratesleast and causes the least discoloration on the walls of the tube. TheCrookes tube contains a very high vacuum,—one millionth degree ofatmosi^heric pressure. In order to exhaust these tubes approximatelythey are subjected to the action of mercury pumps. When a sufficientvacuum has been obtained, the small glass tube that projects from,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectradiolo, bookyear1910
