Page:Popular Science Monthly Volume 63.djvu/366

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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. 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:

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 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