Troiubridge — Magnetic Field to X-ray Tubes. 143 



Art. XX. — The Application of a Longitudinal Magnetic 

 Field to X-ray Tubes ; by John Trowbridge. 



In an article on the Magnetic Field and Electric Discharges 

 in the Proceedings of the American Academy (vol. xli, No. 28) 

 I stated that the application of a longitudinal magnetic field 

 at the anode might form a useful 

 method of concentrating the cathode 

 rays. Since this article was written 

 I have studied the subject more care- 

 fully and have devised a safe, and 

 practical method, which is analogous 

 to that I have described in the article 

 on the phosphorescence of the canal 

 rays, published in the present number 

 of this Journal (p. 141). 



The form of tube is shown in fig. 1 : 

 A is an iron disc anode with a perfo- 

 ration at its middle ; 8 is a solenoid 

 which can be adjusted along an appen- 

 dix to the X-ray bulb ; F is the usual 

 focus plane of polished platinum. Op- 

 posite this focal plane the glass is 

 blown thin to permit the egress of the 

 X-rays. The cathode beam is brought 

 to a focus at F by adjustment of the 

 longitudinal field of the solenoid. The 

 dimensions of the apparatus are as 

 follows : 



Diameter of the spherical bulb, 10 cm ; 

 distance between the concave alumi- 

 nium cathode and the iron disc anode, 6 cm ; length of the 

 cylindrical appendix containing the focal plane, 10 cm ; internal 

 diameter of the cylindrical appendix approximately 3 cm . The 

 outer diameter of the solenoid was 10 cm , the internal diameter 

 3 em , length 4 cm . There were 10 layers of No. 12 wire Brown and 

 Sharpe gauge. The solenoid was excited by two or five storage 

 cells. A narrower appendix and a smaller bulb opposite the 

 focal plane would give a stronger field with less current. 



When the cathode stream is made to converge by the solen- 

 oid on the focal plane F, the intensity of the X-rays is increased 

 in a marked manner. Judging the intensity by the distance 

 at which equal intensity is obtained with and without the mag- 

 netic field, J have more than doubled the intensity of the X-rays 

 by the application of the field. The method has the advantage 



