of Electricity through Rarefied Air. 9 



numbers, we get the deflections produced by the electromotive 

 force existing in the spark. 



The resistance of a column of gas being independent of the 

 cross section of the column, butjbeing necessarily proportional 

 to its length, the resistance of the spark may be expressed by 

 i\l, in which term i\ denotes the resistance of unit length, 

 and I the length. If we represent by e the electromotive force 

 of the spark, and by m the resistance of the rest of the closed 

 circuit, the deflections obtained must, according to what pre- 

 cedes, be (after deducting one scale-division) proportional to 



p — " 1*1 1 



— . The experiments recapitulated above show that the 



difference e—i\l diminishes when the pressure of the air falls 

 from 562 to about 29 millim., but that afterwards it increases 

 again in proportion as the pressure descends to 2 millim. 

 Now the preceding experiments show only the way in which 

 the difference between e and r x l varies with the pressure, and 

 not the modifications undergone by each of these quantities. 



Before proceeding further, there is reason to pay attention 

 to the following circumstances: — When an electric spark is 

 formed between metal electrodes in air of great density, the 

 temperature of the circuit of the spark rises, as is known, to 

 such a degree that the metals are carried in the gaseous state 

 from one electrode to the other. The air is therefore consi- 

 derably heated, and mixed besides with metallic vapours. 

 Now we know from Becquerel's researches, confirmed by Hit- 

 torf's experiments, that at high temperature gases become 

 relatively good conductors. It is consequently impossible to 

 deduce, from experiments performed at a considerable density 

 of the air, what would have been the value of its conductivity 

 at the same density if the air had kept its temperature un- 

 changed. With a greater density of the air, too, the elec- 

 trodes are torn by the current; and as it is then very probable 

 (as I have endeavoured to demonstrate in my first researches 

 upon the electromotive force in question) that e in part de- 

 pends on this, neither is it possible to draw, from experiments 

 made with a considerable density of the air, any sure conclu- 

 sions concerning the dependence of that force on the density 

 of the air when the tearing above mentioned does not take 

 place. If, on the contrary, the air is highly rarefied, it, as 

 perfectly reliable experiments have proved, is but slightly 

 warmed by the electric current, and the electrodes remain 

 intact or are but slightly attacked by the current. Therefore, 

 in order to determine the dependence in which the electromo- 

 tive force and the resistance stand relatively to the density of 



