258 Prof. J. A. Fleming on the Pouhen Arc as a 



b 



burns if incautiously touched. They make a rushing or 

 whistling sound and are of a strong blue colour. 



I£ the inductance in the condenser circuit is varied even a 

 little, the brushes disappear. The above described helix 

 considered simply as an open circuit oscillator, has a natural 

 time period of electrical oscillation of very nearly 5 one- 

 millionths of a second, or a frequency of 0*197 x 10 6 . This 

 value had been exactly determined in previous experiments 

 with it*. As the value of the capacity in the condenser circuit 

 is 0*0029 microfarad, and the total inductance in series with 

 it 215,000 cms., the time period T of the condenser circuit is 

 given by : 



T 7 0-0029 x 2150 00 = 5 pl 



V 5-033 xlO 6 10 6 J ' 



and this shows that to obtain the brush-discharge effect the 

 condenser circuit has to be exactly tuned to the period of the 

 fundamental electrical oscillation of the helix. 



The effect of these oscillations in the wire is to create an 

 intense electric field round the helix extending in all direc- 

 tions for a distance of nearly a metre. Vacuum-tubes of all 

 kinds glow brilliantly when brought near the helix. Espe- 

 cially brilliant is the glow in the form of Neon tube I employ 

 with my cymometer, which, as I showed some three years 

 ago, is extremely sensitive to a high-frequency electric field. 

 The form of tube I use is of the spectrum type, the straight 

 part being a glass tube with a bore of 1 mm., and the 

 enlarged ends cylinders of about 1 cm. diameter and 5 cms. 



lon s- 



The beautiful discovery made some time ago by Sir James 

 Dewar that Neon and other rare atmospheric gases can be 

 separated from the commoner constituents of air by means of 

 charcoal cooled to very low temperature, renders it possible 

 to fill spectrum vacuum-tubes with nearly pure Neon, and 

 such tubes are of very great utility in high-frequency 

 research. When filled with Neon at the proper pressure they 

 glow with an intensely brilliant orange-red light in a high- 

 frequency field. If one of these tubes is held near the helix 

 when in action, it shows by its glow that there is an electric 

 field increasing in intensity all the way up the spiral to the 

 free end. 



But more, the Neon tube shows us that the oscillations are 

 not perfectly continuous. If the tube is waved rapidly to 

 and fro in front of the spiral, by the persistence of vision its 



* See J. A. Fleming, " On the Propagation of Electric Waves along 

 Spiral Wires," Phil. Mag. ser. 6, vol. viii. p. 434 (October 1904). 



