362 POPULAR SCIENCE MONTHLY. 



HERTZIAN WAVE WIEELESS TELEGRAPHY. III. 



By Dr. J. A. FLEMING, F.R.S., 



PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON. 



WE have to consider in connection with this part of the subject the 

 dielectric strength of air under different pressures and for dif- 

 ferent thicknesses. It was shown by Lord Kelvin, in 1860, that the 

 dielectric strength of very thin layers of air is greater than that of thick 

 layers.* The electric force, reckoned in volts per centimeter, required 

 to pierce a thickness of air from two to ten millimeters in thickness, at 

 atmospheric pressure, may be taken at 30,000 volts per centimeter. 

 The same force in electrostatic units is represented by the number 100, 

 since a gradient of 300 volts per centimeter corresponds to a force of 

 one electrostatic unit. It appears also that for air and other gases, 

 there is a certain minimum voltage (approximately 400 volts) below 

 which no discharge takes place, however near the conducting surfaces 

 may be approximated. In this particular practical application, how- 

 ever, we are only concerned with spark lengths which are measured in 

 millimeters or centimeters, lying say between one or two millimeters 

 and five or six centimeters. Over this range of spark length we shall 

 not generally be wrong in reckoning the voltage required to produce 

 a spark between metal balls in air at the ordinary pressure to be given 

 by the rule : 



Disruptive voltage = 3000 X spark gap length in millimeters. 



If, however, the air pressure is increased above the normal by inclu- 

 ding the spark balls in a vessel in which air can be compressed, then 

 the spark length corresponding to a given potential difference very 

 rapidly decreases. Mr. F. J. Jervis-Smithf found that by increasing 

 the air pressure from one atmosphere to two atmospheres round a pair 

 of spark balls, he reduced the spark length given by a certain voltage 

 from 2.5 em. to 0.75 cm. 



Professor R. A. Fessenden has also made some interesting obser- 

 vations on the effect of using compressed air round spark gaps. He 



* See Proc. Roy. 80c., London, February 23 and April 12, 1860; or reprint 

 of papers on electrostatics and magnetism, p. 247. 



t See Phil. Mag., August, 1902, Vol. IV., p. 224, 6th Series. Mr. Jervis- 

 Smith has also described an experiment to show how much the use of com- 

 pressed air round a spark gap is of advantage in working an ordinary Tesla 

 coil. In liis British specification No. 12,039 of 1896, Mr. Marconi had long 

 previously mentioned the use of compressed air round the spark gap. 



