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Mr. A. Schuster. 



Definition of Unipolar Conductivity. 



An electrode often discharges electricity of one kind more freely 

 than that of the opposite kind. The phenomenon has been called that 

 of unipolar conductivity, but we evidently require some understand- 

 ing as to when we shall call a gas or an electrode positively unipolar. 

 If a positive charge on an electrode becomes more quickly dissipated 

 than a negative charge, we may either say that the electrode 

 discharges positive electricity more rapidly into the gas, or that the 

 gas discharges negative electricity more rapidly into the electrode. 

 Some confusion may be caused by the fact that different writers have 

 looked at the question from different sides and have not adopted 

 a uniform nomenclature. I shall call an electrode positively unipolar 

 if it discharges positive electricity more freely into the gas than 

 negative electricity.* A great part of the following paper will deal 

 with the circumstances under which the exchange of an ion between 

 the conductor and the molecules or atoms takes place, and it will be 

 shown how the peculiar behaviour of gases may be explained. This 

 I consider an important step in support of the theory. 



Such expressions as " surface resistance " or " counter electro- 

 motive force " have often been employed in cases where they seem to 

 me to be quite unsuitable, and I shall therefore avoid them altogether ; 

 but I may occasionally briefly speak of an impediment to the passage 

 of electricity at an electrode, meaning that a rapid fall of potential at 

 that electrode is required to introduce or maintain the discharge. 



Circumstances under which the Discharge takes place. 



It will be convenient to divide the subject into two main parts : 

 the one dealing with the circumstances under which a discharge can 

 take place, and the other with the phenomena of the discharge itself. 

 It is well known that, if one of two conductors, separated by a gas, 

 is electrified while the other is connected to earth, the gas acts as a 

 perfect insulator as long as the difference of potential between them 

 does not exceed a certain value ; if it does, the dielectric strength of 

 the medium breaks down. 



It is a matter of some doubt whether, if a spark passes between two 

 conductors of different form, as for instance between a sphere and a 

 plane or between a point and a sphere, the striking distance is 

 different in the two directions. There probably is some difference of 

 this nature in favour of a more easy escape of positive electricity, 



* Elster and Greitel in a recent publication have adopted a different nomencla- 

 ture. They say that a glowing platinum wire in oxygen discharges negative 

 electricity more easily than positive, meaning that the electrode allows a free pas- 

 sage to negative electricity from the gas to the electrode (' Wiedemann, Annalen,' 

 vol. 38 (1889), p. 39). 



