Chapter 3 Part 16
A tube under such conditions is not reliable for photo- graphic exposures, and rapidly suffers in quality in the manner described in the section on ‘Changes during To check off these inverse currents some arrangement is frequently interposed in the secondary circuit between the coil and the anode of the X-ray tube, and for the best photo- graphic effect this should always be done. Such check is necessary with high voltage, with a rectified alternating current, or with a very rapid series of interruptions.
A Villard’s valve-tube, or ‘ soupape/ is the usual piece of apparatus so employed. This consists, as shown in Fig. 38, of a vacuum-tube of moderate degree of exhaustion, having one end drawn out as a slender prolongation of the central space. Into the main space projects a terminal of thick aluminium wire in the form of a corkscrew; and in the farthest part of the prolongation is the second terminal, formed by a slender rod of aluminium. So long as the larger corkscrew-shaped terminal acts as a kathode the tube conducts easily, but to currents in the opposite direction it offers a high resistance. If this tube be placed in proper relation to the X-ray tube, it will be readily seen how it will oppose the passage of the inverse currents described, whilst allowing easy passage to the direct currents desired for use. In series with an X-ray tube the correct setting may be remembered by noting that platinum alternates with aluminium. The platinum anode of the X-ray tube should of course be towards the positive pole of the induction-coil, experience in working installations also testifies to the advantage of the device. A vacuum regulator should always be attached to the valve-tube, otherwise needless resistance may be opposed to the current.
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Tom Thym on December 14th 2009 in x-ray
