of Electricity through Hot Gases. 359 



rature, I have during the last year made a series of experi- 

 ments on the changes which take place in the electrical 

 properties of gases when they are heated. 



Before describing these experiments I will sketch briefly 

 the electrical properties of a gas, such as air, at the tempera- 

 ture of about 16° C. In order to get electricity through the 

 air at atmospheric pressure, the electromotive intensity has to 

 exceed 30,000 volts per centimetre ; as the pressure of the 

 air diminishes, the electromotive intensity required to produce 

 discharge diminishes almost exactly in the same proportion. 

 This goes on until the electric strength of the air reaches a 

 minimum, after which any further diminution in the pressure 

 causes the electric strength to increase; the pressure at which 

 the electric strength is a minimum depending on the size and 

 shape of the vessel in which the air is contained. 



These results are, I think, in accordance with the view of 

 the electric discharge through gases which I gave in the Philo- 

 sophical Magazine for June 1883, and which, I think, derives 

 great support from some of the experiments described below. 

 According to this view, the provision of a supply of atoms by 

 the splitting up of the molecules is the essential accompani- 

 ment of the electric discharge through gases. We may regard 

 the atoms in the molecule as being in oppositely polarized 

 states ; one atom behaving as if it were charged with a quantity 

 of positive electricity, the other as if it were charged with an 

 equal quantity of negative. When the atoms are together 

 in the molecule they neutralize each other's action at points 

 outside the molecule, which behaves as if it were electrically 

 neutral ; but as soon as the atoms separate, since each one is 

 essentially polarized, the gas acquires energetic electrical pro- 

 perties, and by the motion of its atoms electricity can be carried 

 from one part of the gas to another. The ease with which 

 the gas can be made to conduct electricity depends upon the 

 ease with which its molecules can be split up into atoms. 



Let us consider what, according to this theory, happens 

 when a spark passes between two parallel plates. In some 

 one or more of the molecules near the negative electrode the 

 atoms separate, the positively charged atom goes to the nega- 

 tive electrode and the negative atom is repelled. If F is the 

 value of the electromotive intensity between the plates, d the 

 distance of the molecule which is ultimately split up from 

 the electrode, the work done by the electric field on the posi- 

 tive atom when it reaches the negative electrode is Yde, where 

 e is the charge on the atom ; the work done in the same time 

 on the negative atom will not be greater than this, so that 

 the whole work done on the atoms in the molecule will not 



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