THE PHENOMENA OF ELECTROSTATICS. 225 



should be approximately proportional to the pressure. This is, 

 in fact, the case. Thus, the dielectric strength of air at normal 

 atmospheric pressure is about 20,000 volts per centimeter, at a 

 pressure of 10 atmospheres the strength is about 200,000 volts 

 per centimeter, and at a pressure of o. I atmosphere, the dielectric 

 strength is about 2,000 volts per centimeter. The dielectric 

 strength of air reaches a minimum, however, at a pressure of 

 about 2 millimeters of mercury and increases when the pressure 

 falls below this value. An electromotive force, sufficient to pro- 

 duce a spark J of an inch long in air at atmospheric pressure, 

 will produce a discharge through 1 8 or 20 inches of air at 2 

 millimeters pressure. 



The idea of dielectric strength is based on the assumption that 

 the electromotive force required to produce a discharge is propor- 

 tional to the length of the spark, so that the quotient, volts divided 

 by spark length, may be a constant. This is only approximately 

 true in gases under moderate or high pressure, and when the 

 pressure is very low a greater electromotive force is required to 

 strike across a short gap than is required to strike across a long 

 gap. This curious behavior of gas at low pressure is illustrated 

 by a famous experiment due to Hittorf. Two electrodes were 

 sealed into the walls of 

 two glass bulbs and the 

 tips of the electrodes were 

 one millimeter apart, as 

 shown in Fig. 158. The 

 two bulbs were connected 

 together by a spiral tube 

 375 centimeters long, and, 

 when the pressure of the 



Fig. 158. 



gas in the bulbs was re- 

 duced to a very low value, the discharge took place through 

 the long tube and not across the one millimeter gap space be- 

 tween the points of the electrodes.* 



* This behavior of a gas at low pressure is fully explained by the atomic theory. 

 See J. J. Thomson's Conduction of Electricity Through Gases, pages 430-527. 

 16 



