Electrical Resistance of Gases. 203 



§2. 

 (a) According to the unitarian theory, the galvanic current 

 in a closed circuit consists in the free sether present in the 

 circuit being set in translatory motion. The intensity of the 

 current is determined by the quantity of asther passing, in the 

 unit of time, through any section of the conductor ; and the 

 velocity of the aether, for equal intensity of current, is inversely 

 proportional to the magnitude of the section. The galvanic 

 current may therefore be compared to the current of an ordi- 

 nary gas in a system of tubes ; and the properties belonging 

 to a current of this latter kind are therefore to be met with, 

 mutatis mutandis, in the aether-current. Thus, let us imagine 

 a tube, of which one half has the section 1, and the section of 

 the other half is n times as great ; let us suppose, secondly, 

 that this tube is filled with a fluid (liquid or gaseous) having a 

 translatory motion impressed by forces acting at one of the 

 extremities of the tube. Now, if we wish to stop or diminish 

 at any point the motion of the fluid by a counterpressure (for 

 instance, by means of a piston or otherwise), in order to attain 

 the same effect we must cause to act in the widest part of the 

 tube n times as much pressure as in the narrowest. The dimi- 

 nution of the velocity of the motion by means of the counter- 

 pressure does not depend on the absolute value of the latter, 

 but on its value in comparison with the unit of section. If 

 the counterpressure upon the unit of section is as strong in 

 the widest part of the tube as in the narrowest, the diminu- 

 tion of the intensity of the current is equal in the two cases. 

 It will always be so, whatever the resistance, provided that 

 the fluid employed possess sufficient fluidity to produce equal 

 pressure in all directions. 



What has just been said finds its direct application in the 

 galvanic current. Whatever opinion may be held upon the 

 nature of electricity, all agree that it is a fluid which the ex- 

 treme mobility of its particles permits to communicate pres- 

 sure in all directions. Gralvanic resistance impedes the motion 

 of electricity; it therefore acts as a pressure in the opposite 

 direction distributed uniformly over all points of the section 

 of the conductor. Now, if two resistances (two wires, for 

 instance, each of a different metal, and with different sections) 

 produce an equal diminution in the intensity of a given cur- 

 rent, their resistance is said to be equal. We know, likewise, 

 in conformity with the foregoing, that the counterpressure 

 opposed by each of them, on the unit of section, to the propa- 

 gation of the current is in like manner equal. It is therefore 

 exclusively the counterpressure on the unit of section that can 

 serve for the determination of the galvanic resistance. This 

 R2 



