648 ELECTRICAL MEASUREMENTS 



ployed this principle in the electrostatic power indicator, or 

 cyclograph, see page 326. 



General Considerations. In order that the tube may oper- 

 ate satisfactorily it is essential that the vacuum be properly 

 adjusted. As the degree of vacuum changes with use of 

 the tube, (usually increasing with the time), some experi- 

 menters, in work of long duration, keep the tube attached to the 

 air pump so that the vacuum may be adjusted at will, while others 

 have used different forms of vacuum regulator. One such device 

 consists of a thin platinum or palladium tube, closed at the outer 

 end and sealed into the vacuum tube. If the vacuum be too high, 

 the outer end of the platinum tube is heated for an instant to a 

 dull red by a spirit lamp; a small amount of hydrogen will then 

 pass into the tube and lower the vacuum. This arrangement 

 gives no means of increasing the vacuum, which is sometimes 

 necessary. 



Trouble may be caused by sparking from the cathode to the 

 glass in its immediate vicinity. Such sparking should be avoided 

 since it causes the cathode ray stream to be unsteady. The glass 

 becomes positively charged, and when the P.D. between the 

 glass and the cathode becomes high enough a discharge occurs. 



These two difficulties are serious and in the past have been 

 sufficient to prevent the cathode ray tube becoming a reliable 

 and convenient instrument, in spite of the fact that it is peculiarly 

 adapted to certain kinds of work. 



These difficulties have to do with the film of gas which adheres 

 to the surfaces within the tube. It has been found that if the 

 tube is exhausted at a temperature of about 350C., and the time 

 of exhaustion is properly adjusted, about one-half hour, a 

 sufficient quantity of the adsorbed gas may be removed so 

 that the vacuum will remain constant during long periods and 

 still leave a sufficient film on the glass so that the charges on it 

 are conducted to the cathode and neutralized. 



Disturbing effects due to stray electromagnetic and electro- 

 static fields must be eliminated, the latter by the use of proper 

 screens. 



Electrostatic tubes with external condenser plates should be 

 rendered non-hygroscopic in the neighborhood of the plates so 

 that the moisture may not screen the cathode ray stream. 



