A Low Voltage Cathode Ray Oscillograph ' 



By J. B. JOHNSON 



Synopsis: A sensitive cathode ray oscillograph is described which 

 operates at the low potential of from 300 to 400 volts. The electron stream 

 comes from a thermionic cathode and is focused by the action of ionized 

 gas in the tube. This gas, at a pressure of a few thousandths of a milli- 

 meter, serves to reduce to 1mm. diameter a spot which would be 1 cm. 

 across in a high vacuum tube. The sensitivity of the tube is such that 

 the deflection of the spot is about 1 mm. per volt applied between deflector 

 plates. When using magnetic deflection, a pair of coils 4 cm. in diameter, 

 placed on the sides of the tube produces a deflection of approximately 1 mm. 

 per ampere-turn of the coils. — Editor. 



A CATHODE ray oscillograph tube operating at a compara- 

 tively low voltage was described by the wTiter some time ago 

 before the American Physical Society.^ Since then, the tube has been 

 further improved and its operation studied so that now both the 

 structure of the tube and the principles which have made the con- 

 struction possible can be described in greater detail. 



In the older types of Braun tubes the electron stream is produced 

 by a high voltage discharge through the residual gas in the tube. 

 This requires a source of steady potential of from 10,000 to 50,000 

 volts, an installation which is expensive, non-portable, and dangerous. 

 In the new type of tube the low voltage operation has been obtained 

 by the use of a Wehnelt cathode as the source of electrons, so that the 

 lower limit of voltage is set by the effect of the electrons on the fluores- 

 cent screen and not by the voltage needed to obtain the electrons. 

 At 300 volts the electrons produce quite bright fluorescence on the 

 screen and the tubes are therefore designed to operate at 300 to 400 

 volts. 



The external appearance of the tube is shown in Fig. 1. The 

 electrodes are located at one end of the pear-shaped bulb, and the 

 fluorescent material is deposited on the inside of the larger, flattened 

 end. The tube is provided with a base which fits into a bayonet 

 socket such as is used for vacuum tubes, and all the connections are 

 made through the base. There are two orthogonal pairs of deflector 

 plates inside the tube for electrostatic deflection, while magnetic 

 deflection is produced by applying a field from the outside. 



The internal structure differs considerably from that of previous 

 forms of Braun tube and it will therefore be described somewhat 

 fully. 



'Also published in the" Journal of the Optical Society of America and Review oj 

 Scientific Instruments, September, 1922. 

 *Phys. Rev. (2), Vol. 17, p. 420, 1921. 



142 



