1892.] to produce a Spark in Air at different Pressures. 107 



FIG. 4. 



and pass above each other, or, which, is the same thing, the curves 

 in fig. 4, showing the relation between potential difference and spark- 

 length for given pressures, also exhibit minima. This effect, though 

 appearing consistently, was not very distinctly marked, but there 

 were indications that it would be more apparent at lower pressures, 

 and, therefore, it was thought advisable to submit it to more direct 

 investigation. For this purpose new apparatus was fitted up, as 

 described below. 



