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



PART II. 



The apparatus used is sketched in fig. 5. Two pairs of plates 

 at different distances apart, but with same air pressure, were 

 arranged in parallel, so that an alternative path was offered to the 

 discharge. The two pairs of plates were enclosed in two small 

 receivers. The lower plate in each rested on an insulating stand ; the 

 upper plate was supported and separated from the lower by small thin 

 discs of ebonite, mica, or glass. The four plates used were plane, and 

 the thickness of the separating discs, as measured by the screw 

 calipers, is taken as the length of the spark. The receivers rested on 

 well-ground brass base plates, into each of which were sealed two 

 narrow glass tubes, containing mercury, and having sealed platinum 

 wires passing to the outside ; flexible wires passed from the discharge- 

 plates and dipped into the mercury in the tubes. One of these 

 tabes served also, in each case, to give connexion with the pumps. 

 Two pumps were used, the water pump for rapid exhaustion to about 

 30 or 40 mm. of mercury, the mercury pump for lower pressures. 

 Pressures of from 2 to 3 mm. of mercury were easily reached, and 

 could be maintained for any length of time. 



FIG. 5. 



To Water Pump. 



To tbo Cells. 



The platinum wires, in connexion with the plates, dipped into 

 mercury cups in the paraffin blocks N x and N 2 ; from these leads 

 passed to the block M, where either one pair of plates, or both, could 

 be connected to the leads passing to the cells. The cells were used 

 in the same manner as before ; the potential difference, however, 

 instead of being reckoned as proportional to the number of cells 

 applied, was measured directly by a cylinder quadrant electrometer. 

 This was used in order to avoid a fresh examination into the uni- 

 formity of the cells. 



