145. DETAIL CONSTRUCTION AND USE OF SINGLE ELECTRODE DISCHARGE TUBES FOR X-RAYS. He pointed out that with two electrodes in a bulb as previously employed by nearly all experimenters, or an internal one in combination with an adjacent external one the E. M. F. applicable was necessarily greatly limited for the reason that the presence of both, or the nearness of any conducting object “had the effect of producing the practicable potential on the cathode.” Consequently he was driven, as he said, to a discharge tube having a single internal electrode, the other one being as far away as required. § 9. In view of his ingenious arrangements of the disruptive coil, and circuits, condensers and static screens for the bulb, he found it immaterial to pay attention to some other details followed by experimenters. For example, it made comparatively little difference in his results whether the electrode was a flat disk or had a concave surface. [Illustration: TESLA’S FIGS. 1 AND 2, REFLECTION AND TRANSMISSION OF X-RAYS BY DIFFERENT SUBSTANCES. § 145 and § 146_a_. ] The form of tube described by Tesla in full, will hereinafter be alluded to as exhibited in the several figures accompanying this description, and it consisted, therefore, of the long tube “_b_” made of very thick glass except at the end opposite the electrode “_e_”, where it was blown thin, p. 149. The electrode was an aluminum disk having a diameter only slightly less than that of the tube and located about one inch beyond the rather long narrow neck _n_, into which the leading-in wire _c_ entered. It is important that a wrapping _w_ be provided around this wire, both inside and outside of the tube. The sealing point was on the side of the neck. An electric screen has been referred to heretofore. It is lettered _s_, and was formed of a coating of bronze paint applied on the glass between the electrode and neck _n_. The screen could be made in other ways, for example, as shown at _s_, Fig. 2, where it consists of an annular disk behind and parallel to the electrode disk. This ring _s_ in Fig. 2. must be placed at the right distance back of the electrode _e_, but just how far can only be determined by experience. The unique service of the screen was that of an automatic system for preventing the vacuum from becoming too high by use. The peculiar action was as follows, namely from time to time, a spark jumped through the wrapping _w_ within the tube to the screen and liberated just about enough gas to maintain the vacuum at an approximately constant degree. Another way in which he was able to guard against too high a vacuum, consisted in extending the wrapping _w_ to such a distance inside of the tube, that the same became heated sufficiently to liberate occluded gases. As to the long length of the leading-in wire within a long neck behind the cathode, Lenard found the same to be valuable in conjunction with a wrapping around the wire. With high potential, a spark, at a