X-Rays, Apparatus and Methods Page 15

produce only a very small secondary discharge. The energy of this secondary discharge may then be immediately increased until the reflector plate shows a dull red spot of about half the size of the nail of the little finger. The tube is now in its most efficient condition, and, with a well constructed and smooth working break, may be run for an hour or two without the slightest attention. The "getting ready," as above described, takes considerably less time than the time it has taken us to describe it. Indeed, in our own work, we do not trouble to make this preliminary adjustment at all, as we make it once for all when we first begin to work with a tube and do not afterward disturb it, merely closing down the switch when starting work.

Life of Tubes. — The life of tubes in general is limited —

(a) By time required for the vacuum to become too great, hence requiring re-exhaustion.

(b) By ability to stand the electrical strains.

(a) Applies to all tubes, so far as we know, save the Bowdoin tubes — probably six or eight hours' continuous working, or the equivalent suffices to incapacitate the average tube. We think that even with the Bowdoin tube there is an evident tendency to eventual very high vacuum, but the time required is quite large, relatively, say ten to fifteen times that operative in the case of other types of tubes.

(b) All well-made tubes (from the glass-blower's standpoint) are about equally liable to this limitation. It is probable that these strains are cumulative in their effects upon the tube, the latter gradually weakening until eventually breakdown takes place with secondary discharges much less than would have been originally required for breakdown.

The Stand. — The stand, shown in Fig. 6, has been devised by us for applied X-ray work. With it the tube may be placed in any position within a cylinder 6 feet high and 6 feet in diameter, thus making it possible to get under or over a patient, no matter what the latter's position. Four rods (two are not shown in the figure) of vulcanized fibre clamp to the stand in any position and carry the lead wires of the tube, thus keeping them away from one another and