102 Mr. J. B. Peace. Potential Difference required [June 16, 



was taken as being proportional to the number of cells. The obser- 

 vations were made for a series of pressures ranging from 20 to 

 300 mm. of mercury. 



When the apparatus was first used, a fixed potential difference was 

 given to the plates, and the pressure gradually lowered until a spark 

 passed. When, however, it was found that, in certain circumstances, 

 a diminution of pressure required an increase of potential difference, 

 this method was abandoned, and that described above was adopted. 



Within the specified range of pressure a considerable number of 

 observations were made in July and August, 1891, for spark-lengths 

 corresponding to 2, 5, 10, 20, 40, 100, 200, and 400 divisions of the 

 screw head. The mean results of these observations are given in 

 the following tables. Each table represents the mean of at least 

 three series of observations, taken at considerable intervals of time, 

 and generally showing very close agreement with each other. 



From the observed potential difference the electrostatic force has 

 been deduced, that is, the potential difference (in electrostatic units) 

 per centimetre of spark-length. The electrostatic force is given in 

 the third column of the following tables, the first column containing 

 the air pressure in millimetres of mercury, and the second the 

 potential difference in volts, while d is the spark-length, as measured 

 above. 



The tables are represented by the curves of figs. 2 and 3, fig. 2> 

 showing the connexion between potential difference and pressure for 

 various constant spark-lengths, and fig. 3 showing the connexion 

 between electrostatic force and pressure for the same spark-lengths. 

 From these are deduced the curves in fig. 4, showing the relation 

 between potential difference and spark-length for various constant 

 pressures. 



