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



300 mm. of mercury, the water-pump which was used, and the 

 stuffing-box through which the sparking distance was adjusted, not 

 admitting of observations at lower pressures. The results of these 

 observations are tabulated below, and appear in the curves in figs. 2, 



3, and 4. 



When these results were examined and compared, it was iound 

 that the curves connecting potential difference and pressure not only 

 exhibited the minima above mentioned, but also crossed each other, 

 showing that at low pressures the shorter spark required the greater 

 potential difference. As this seemed a point of some importance, it 

 was decided to make a more direct investigation, and with this view 

 apparatus was constructed and observations made in April, 1892. 

 This apparatus is described, and the results obtained are recorded in 

 Part II below. The observations were confined to pressures ranging 

 from 2 or 3 up to 50 mm. of mercury, and the results confirmed and 

 extended those formerly obtained. 



PART I. 



A sketch of the apparatus used is given in fig. 1. An inverted 

 bell- jar, communicating with a water-pump, had its mouth ground 

 and fitted with a brass cover, C. This cover carried the attachments 

 of the discharge plates P, P, and the micrometer screw S. The screw 

 had a pitch of ^ inch, and its head was graduated in hundredths of 

 a revolution. A short steel rod, fixed to the sliding frame of the 

 screw and working through a stuffing-box in the centre of the cover 

 C, carried an insulating block, to which the upper discharge plate 

 could be readily attached. The lower plate was fixed to an ebonite 

 disc which was rigidly suspended from the cover C. The plates were 

 of brass, highly polished, and were about 2|- inches in diameter. The 

 lower plate was plane, the upper was slightly convex, having a radius 

 of curvature of about 9 inches. Short flexible wires passed from the 

 plates to insulated terminals passing through the cover. From these 

 terminals leads passed to mercury cups, M, by which connexion 

 could be made, on the one hand with the leads of the telephone T, 

 and, on the other hand, with leads passing to the secondary cells and 

 to the voltmeter. 



The cells were arranged in groups of eighteen in series; for 

 charging, the groups were arranged in parallel ; for spark discharge 

 they were arranged in series, while the leads passing to M could be 

 connected to the terminals of any required number of cells. In the 

 discharge circuit there were also a simple make-and -break key and a 

 veiy high resistance, the latter to prevent the setting up of an arc 

 discharge between the pi ales. This resistance was usually a black- 

 lead line on a strip of ebonite. 





