8 Prof. E. Edlund on the Electrical 



tion. Therefore, to obtain as small a resistance as possible, as 

 large a surface as possible must be given to the negative pole. 

 It appeared moreover that the material composing the surfaces 

 of the poles had an influence upon the amount of the apparent 

 resistance. These observations have been confirmed by those 

 which we shall cite below, and present the interesting feature 

 that the current was produced by an ordinary battery, which 

 was of course not sufficiently powerful to call forth the forma- 

 tion of a voltaic arc. Hittorf has specially confirmed the fact 

 that gases become conductors when heated to redness*. 



Gaugainf made the following observation : — If the current 

 of a Ruhmkorff apparatus passes through the air in a glass 

 tube in which two platinum wires are inserted as electrodes, a 

 galvanometer intercalated in the circuit shows that the inten- 

 sity of the current increases in proportion as the air is rarefied, 

 until the rarefaction has reached a certain limit, but that the 

 intensity begins to diminish if the rarefaction be carried beyond 

 that limit. The degree of rarefaction at which the intensity 

 of the current arrives at its maximum depends on a multitude 

 of circumstances — such as the distance between the electrodes, 

 the size of their surfaces, the width of the tube, &c. In general 

 the degree of rarefaction necessary for the maximum of inten- 

 sity of the current has increased in the same measure as the 

 augmentation of surface of the negative electrode ; when, on 

 the contrary, the surface of the latter was small, the rarefac- 

 tion of the gas did not need to be carried so far in order to 

 bring about the maximum of intensity of the current. 



Morren has found in his experiments the observation con- 

 firmed that the intensity of the current augments with the 

 rarefaction of the gas up to a certain limit, past which it begins 

 to diminish if the rarefaction is carried further %. He em- 

 ployed for this purpose a Ruhmkorff apparatus, and a glass 

 tube of 29 millim. internal diameter and 45 centim. length, in 

 which the distance between the soldered aluminium electrodes 

 amounted to 24 centim. Hydrogen commenced giving evi- 

 dent deflections when the rarefaction had reached a pressure 

 of 74 millim. of mercury ; the deflections presented themselves 

 at 39 millim. for carbonic acid, at 29 for atmospheric air, and 

 at 23 for nitrogen and oxygen. Of course these figures 

 possess only a relative importance and are of value only for 

 the conditions under which the experiments took place. The 

 maximum intensity of the current presented itself, for the 

 first-mentioned gas, at a pressure of 1 millim., for oxygen and 



* Pogg. Ann. Jubelband (1874), p. 434. 



t Comptes Rendus, xli. p. 152 (1855). 



j Ann, de Chimie vt de Plnjsique, (4) iv. p. 325 (1865). 



