368 Prof. Pollock and Mr. Ranclaud on the 



Gonradty arc-lamp carbon, turned down at. the end as shown 

 in the figure. 



The temperature of the heated carbon was found with the 

 aid of a Holborn-Kurlbaum optical pyrometer, calibrated at 

 the Reichsanstalt. The temperatures are given as observed, 

 they are therefore in all cases the equivalent black body 

 values. 



To get an estimate of the temperature of the cool carbon 

 when currents were measured, the junction of a thermo- 

 electric thermometer of platinum and platinum-iridium wires, 

 0*5 millimetre in diameter, was placed in the position of the 

 end of the carbon rod, the rod being temporarily removed. 

 The temperature given by the thermometer, under any cir- 

 cumstances as to distance of the junction from the heated 

 cylinder and length of time in position close to it, is 

 considered to be that of the end of the carbon under similar 

 conditions. 



3. Flow of Negative Electricity from the Hot Carbon. 



The general form of the relation, without reference to 

 exact scale, between potential - difference and current for 

 temperatures of the hot rod in the neighbourhood of 1800° C, 

 is shown in fig. 3, where ordinates represent currents, and 

 abscissae potentials of the hot relatively to that of the cool 

 carbon. 



For the flow of positive electricity from the hot carbon it 

 is sufficient to say that any increase, above the value for zero 

 current, in the potential of the hot carbon, gives a much 

 smaller current than a decrease of the same amount. This 

 smaller current, however, under the circumstances of our 

 experiments, is not the definite measure of the stream of 

 positive ions from the hot carbon ; it is the resultant of 

 such a stream and one of negative ions from the cooler 

 electrode. 



The current of negative electricity from the hot carbon 

 depends on the temperature of the carbon, on the potential- 

 difference between the electrodes, and on the distance sepa- 

 rating them. At low voltages the current is of the order of 

 a milliampere and is not accompanied by any luminosity. 

 As the potential-difference increases a critical value, depend- 

 ing on the temperature and on the distance between the 

 carbons, is reached at which an arc forms, and the current 

 jumps instantly from milliamperes to amperes ; the critical 

 points are shown at X in the figures. 



To exhibit the form of the connexion between potential- 

 difference and current, for the non-luminous regime when 



