Nikola Tesla Articles
X-Rays, Apparatus and Methods Page 14
Journal of the Franklin Institute - March 1st, 1897
ing over a few minutes' exposure or running of the tube one of the above methods becomes necessary. In any case, eventual, almost complete, exhaustion seems to be the rule, and the tube must then be returned to the makers and re-exhausted.
"Blackening" is another cause of decreased efficiency in the tube. It is due to deposition of platinum upon the inner surface of the tube.
The "Bowdoin" tube, shown in Fig. 5, was devised after a considerable amount of experimental work, by Profs. Robinson and Hutchins, of Bowdoin College. In general design it does not differ essentially from many other forms of focus tube upon the market. The two respects in which the tube does differ from other tubes are, first, in the arrangement of electrodes, the reflector plate not being connected to the coil at all, but merely acting as an obstacle to the bombarding rays, while the anode is a disc at the end of the tube away from the cathode. This arrangement is found to perfectly and entirely prevent blackening, no matter how long the tube may be used.
The most important feature of this tube is, however, the presence upon its interior surface of a fluorescent material which has been fused into the surface of the glass.* As the tube is used, this material, either by the bombardment or heating, or both, gives off just enough air or vapor to make up for the occlusion due to the natural working of the tube. The vacuum, therefore, tends to remain always constant. Although it seems almost absurd that one could put in just enough of this material to make up for the gas otherwise lost, yet it is a fact that these tubes remain practically unchanged in vacuum for a very long period.
In use the tube requires no "nursing" whatever. It should always have, to start with, a parallel spark gap, which should be quite small, say 1 inch or 1 1/2 inch, and either the primary current or the vibrator tension adjusted so as to
* The fluorescent quality is not supposed to have any effect upon the result; it merely happens that the particular substance found most effective is fluorescent.
