646 ELECTRICAL MEASUREMENTS 



wave form appears. This is the common arrangement for 

 visual observations. 



As the cathode stream is without inertia, the deflection will 

 follow waves of the highest frequency without error.* In this 

 the Braun tube is unique among devices for tracing wave forms. 



As the cathode stream is deflected as readily in one direction 

 as in another, the spot of light will take up a position dependent 

 not only on the magnitude but on the direction of the resultant 

 field due to any system of magnetizing coils. This is a second 

 unique feature of the instrument. 



Permanent Records by Braun Tubes. To obtain a permanent 

 record it is necessary to introduce a time coordinate in some 

 manner. The simplest method is to photograph the fluorescent 

 spot on a film carried by a synchronously rotating drum ; the wave 



C- (.MICROFARAD COMPLETE PERIOD 

 L-.04A HENRY .0012 SEC. 



R- 9. OHMS 



FIG. 408. Showing oscillatory current due to the discharge of a condenser 

 through an inductive resistance. Taken with Braun tube. 



then appears on the film in rectangular coordinates. The curve 

 shown in Fig. 408 was taken in this manner. As the photo- 

 graphic intensity of the fluorescent spot is not great, the Braun 

 tube is adapted to recording periodic phenomena only. To ob- 

 tain the curve shown in Fig. 408 the circuit was made and 

 broken by a commutator attached to the axle of the drum and the 

 spot of light traversed the same path on the film many hundred 

 times. The inability to obtain records by a single traverse of 

 the image of the spot over the photographic surface is a serious 

 drawback to this form of oscillograph. Another limitation is 

 apparent from the figure. The width of the line which is traced 

 is considerable when compared with the amplitude of the curve. 



* See paper by DR. E. L. CHAFFEE, Proc. American Academy of Arts 

 and Sciences, vol. 47, 1911-12, p. 267. 



