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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