1892.] Potential Difference required to produce a Spark. 99 



are afc least as accurate as those of Ludwig's method, while it is far 

 easier to carry out, and occupies much less time. 



VI. " On the Potential Difference required to produce a Spark 

 between two Parallel Plates in Air at Different Pressures." 

 By J. B. PEACE, M.A., Fellow of Emmanuel College, Cam- 

 bridge. Communicated by Professor J. J. THOMSON, F.R.S. 

 Received May 24, 1892. 



Introductory. 



The following is an account of an investigation into the relations 

 between potential difference, spark-length, and pressure when dis- 

 charge takes place between two parallel plates in air. The in- 

 vestigation was undertaken at the suggestion of Professor J. J. 

 Thomson, and has been carried out in the Cavendish Laboratory in 

 Cambridge. 



Preliminary experiments were made in July and August, 1890, 

 in which a small Wimshurst machine was used, and the potential 

 difference was measured by one of Lord Kelvin's vertical electro- 

 static voltmeters. For sparks of some length fairly consistent 

 results were obtained, but in the case of short sparks accurate 

 measurement of the potential difference was difficult. Use was then 

 made of a large number of small secondary cells, and, at the same 

 time, more accurate measurements of the spark-length were made. 

 At first about 500 cells were used, so that steady potential differences, 

 ranging up to about 1,000 volts, could be maintained, and sparking 

 differences up to about 0*1 mm. were examined. The results of these 

 experiments showed very fair agreement with those of other observers ; 

 they were sufficiently consistent among themselves to show that the 

 cells could be relied on. More cells were therefore set up, and all 

 were carefully tested as to equality, rate of fall of charge, &c. 



Apparatus was then constructed by which discharge at different 

 pressures could be examined. Description of this apparatus, which 

 is sketched in fig. 1, and an account of the method and results of 

 observations made with it in July and August, 1891, are given in 

 Part I below. At first some difficulty was found in making the bell- 

 jar and its connexions sufficiently air-tight, but when this was over- 

 come an effect similar to that observed by De la Rue and Miiller in 

 discharge in exhausted tubes became apparent ; that is, it was found 

 that, as the pressure diminished, the potential difference required to 

 produce a spark across a given distance fell to a minimum value, and 

 then began to increase rapidly. To examine this effect more fully, 

 the observations were confined to pressures ranging from 20 to 



]i 2 



