THE ELECTRON THEORY. 279 



in order to receive enough energy to enable it to ionize an atom of 

 oxygen or nitrogen, and an ion must fall freely through a differ- 

 ence of potential of about 440 volts in order to receive enough 

 energy to enable it to ionize an atom of oxygen or nitrogen. 



139. The electric spark in a gas. When a gas is subjected to 

 an electric field of which the intensity is sufficient to cause both* 

 the electrons and the positive ions to ionize the gas, an extremely 

 rapid and unlimited increase in the number of electrons and ions 

 takes place, and the result is the production of an electric spark. 

 The mean free path of the positive ions in a gas is inversely pro- 

 portional to the pressure of the gas so that the electric strength 

 of a gas 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 32,000 volts per centimeter, 

 at a pressure of 10 atmospheres the strength is about 320,000 

 volts per centimeter, and at a pressure of o.i atmosphere the 

 dielectric strength is about 3,200 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 produce a spark 1/8 of an inch long in air at atmos- 

 pheric 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 

 proportional to the length of the spark, so that the quotient, volts 

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



* When the intensity of an electric field is sufficient to cause only the electrons 

 to ionize the gas, then all of the electrons form new ions and new electrons as they 

 travel towards the positive electrode, and when they reach the positive electrode 

 the action ceases except for the occasional formation of a stray electron by outside 

 influences. When the electric field is sufficiently intense to cause electrons and 

 positive ions both to produce ionization, then new ions and electrons are formed 

 everywhere between the electrodes, and the number of free ions and electrons in- 

 creases indefinitely. It is a well-known fact that an electric field must continue 

 to act for an appreciable time before a spark is produced. 



