CHAFFEE. — IMPACT EXCITATION OF ELECTRIC OSCILLATIONS. 289 



feet of running water in glass tubes. In this investigation the Braun 

 tubes were operated on Professor Trowbridge's large 20,000 cell battery, 

 which has proved of the greatest value in this and other work. Usually 

 not over 20,000 volts were used, that being sufficient, under proper 

 adjustment of vacuum, to give a very intense and homogeneous 

 cathode beam. 



The proper adjustment of the vacuum is of the utmost importance, 

 and depends entirely upon conditions. For the greatest intensity of 

 the spot and, at the same time, the greatest sensitiveness, the pressure 

 inside the tube should be as high as possible consistent with a clearly 

 defined spot. After some experience, the appearance of the discharge 

 about the cathode serves as the best indicator as to the exhaustion. 

 The best results are usually obtained when the luminous stream from 

 the center of the cathode is about 1 mm. in diameter, although in 

 special cases a higher or perhaps a lower exhaustion is desirable. The 

 vacuum, however, changes when using large currents, making it neces- 

 sary that the tubes be in connection with a mercury pump. The Gaede 

 rotary pump was used in this investigation. 



If a pump in connection with the tube is not convenient, a vacuum 

 regulator, containing potassium chlorate or some other substance such 

 as is used on X-ray bulbs, is suggested for lowering the vacuum when 

 necessary. 



In order to eliminate the deflection of the beam by the earth's mag- 

 netic field, the tubes are mounted with their axes parallel to the earth's 

 magnetic force, which mounting gives the tubes an inclination of about 

 70° with the horizontal. The camera, for photographing the oscillo- 

 graphs, was directed downward, viewing the upper or front surface of 

 the fluorescent screen. Plate 1 a, shows the general arrangement of 

 apparatus. 



The deflecting coils, when used with high frequency current, can, of 

 course, contain no iron. The coils, which were used in this investiga- 

 tion for the magnetic deflection of the cathode beam, are composed of 

 from two to sixteen rectangular turns of copper wire, .325 cm. in diam- 

 eter, half of the turns being on either side of the tube. The turns are 

 about 7 or 8 cm. in width, and from 10 to 12 cm. in length. A nearly 

 uniform field within the coil is secured by making the width of the 

 rectangular turns /y/2 times the distance between the planes of the two 

 parts of the coil. 



When using high irequencies, the difference in potential between 

 two parts of the same coil, due to inductance, is sufficient to ca.use very 

 large electrostatic deflections of the cathode beam. This disturbance 

 is most effectually eliminated by surrounding the tube inside the coil 



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