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



FIG. 7. 



The curve given in fig. 8 represents corresponding values of air 

 pressure and spark-length for minimum potential difference. The 

 points marked on this curve by a dot are taken from the results of 

 Part I, those marked by a cross from the results of Part II. The 

 long straight portions of the curve cannot be regarded as quite 

 accurate, for the following reasons : That portion which lies close 

 to the vertical (pressure) axis refers to the extremely short spark- 

 lengths of Part I, in measuring which, an error in the screw-reading 

 or in the zero adjustment might be a considerable fraction of the 



