Discharge of Electricity from Glowing Carbon. 369 1 



the hot carbon is negative with respect to the cool one, the 

 relations between them, for distances separating the carbons 

 of 1, 2, and 3 millimetres, are shown in figs. 4, 5, and 6. 

 The final portions have been drawn vertical, the abscissas of 

 the vertical parts being the values of the voltages at which 

 arcing occurred. The numbers attached to the curves are 

 the values of the temperatures of the hot and cool carbons 

 respectively. 



4. Critical Values. 



The phenomenon of an abrupt change from a non -luminous 

 to a luminous discharge is described by Professor Thomson* 

 for the case of a gas at low pressure. A similar sudden 

 change takes place in air at normal pressure, and under the 

 circumstances of our experiments, the alteration, from the one 

 regime of current flow to the other, can be brought about by 

 a change in the potential-difference between the carbons of 

 about a volt. 



The currents just before arcing occurs can be deduced by 

 graphical extrapolation, as shown in the above diagrams; 

 the approximate values so found are collected in Table I. 



Table I. 



Distance 



Temperature. 



Critical 



Current 



between 







Voltage 



just before ; 



Carbons. 



Hot Carbon. 



Cool Carbon. 



for arcing. 



arcing. 









volts. 



milliamperes. 



1 mm. 



1800° C. 



1360° C. 



435 



237 



>> 



1680 



1240 



550 



250 



., 



1550 



1100 



08-5 



266 



» 



1415 



920 



830 



282 



2 mm. 



1800 



1120 



530 



1-85 



,, 



1(580 



980 



(•,(•)(» 



1-95 



!) 



l ;>:>( » 



860 



81-0 



2-07 





1415 



650 



97 



220 



3 mm. 



20G0 



1070 



395 



097 



„ 



1800 



880 



7(H) 



112 





1680 



750 



85-0 



120 



The relation, as given by our observations, between the 

 temperature of the hot carbon and the critical value of the 

 potential-difference between the electrodes necessary to start 

 arcing, is shown in fig. 7 for distances of 1, 2, and 3 millimetres 

 between the carbons. 



* Thomson, lac. cit. 



