370 



Prof. Pollock and Mr. Ranclaud on the 



All the observations given in this paper, with the exception 

 of those in the following table, refer to the case where the hot 

 and cool carbons are in the same horizontal plane. The 

 potential-difference necessary to start arcing, however, depends 

 very considerably on the position of the hot relatively to the 

 cool carbon, and we give in Table II. measurements of this 

 quantity with the hot carbon vertically below, and with it 

 vertically above, the cool positive ; in the lower part of the 

 table are given the values of the critical voltage with the hot 

 carbon vertically below the cool one, and with it in the same 

 horizontal plane as the other electrode. 



Table II. 



Distance 

 between 

 Carbons. 



Temperature. 



Critical voltage for arcing. 



Difference. 



Hot carbon. 



Cool carbon. 



Hot carbon 

 below. 



Hot carbon 

 above. 



3 mm. 



1930° C. 



1420° C. 

 346 



volts. 

 56-0 



volts. 

 74-0 



volts. 

 18-0 



)5 



2110 



1510 

 650 



39-5 



54-5 



15-0 



» 



2230 



1550 

 860 



26-5 



390 



12-5 



»» 



2460 



1620 

 1240 



12-5 



20-5 



8-0 



1 mm. 



1930 



5) 



1580 

 1280 



36 



440 



8-0 



2 mm. 



5J 



1530 

 840 



42-0 



52-0 



100 



3 mm. 



5) 





1420 

 346 



56-0 



74-0 



18-0 



1 mm. 



1930 



1580 

 1500 



36-0 



Carbons in 

 same hori- 

 zontal plane. 



42 



60 



2 mm. 



» 



1530 

 1200 



42-0 



460 



40 



3 mm. 



>> 



» 



1420 

 1000 



56-0 



59-5 



3-5 



>> 



2230 



1550 

 1240 



265 



29-5 



30 



i 



It is seen from the table that the potential-difference 



