| HERTZIAN WAVE WIRELESS 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 different 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.[1] 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, however, 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 spark gap length in millimeters.
If, however, the air pressure is increased above the normal by including 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-Smith[2] 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 observations 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.
- ↑ 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 compressed air round a spark gap is of advantage in working an ordinary Tesla coil. In his British specification No. 12,039 of 1896, Mr. Marconi had long previously mentioned the use of compressed air round the spark gap.
