NICHOLS. 



THE VISIBLE RADIATION FROM CARBON. 



79 



that this correction would be a small one, but attempts to measure in a 

 similar manner the temperature of the acetylene flame indicated that the 

 loss of heat from this source was by no means to be neglected. These 

 attempts are described in a subsequent section of this paper. 



The numerical value of this correction was accordingly determined 

 by direct experiment in the following manner. Thermo-elements 

 drawn from the same pieces of wire but differing considerably in 

 diameter were prepared. These were inserted two at a time in holes 

 on opposite sides of a carbon rod and the rod was brought to incandes- 

 cence by means of the current. The temperatures reached by these 

 junctions were compared by means of the potentiometer, and a curve 

 was plotted showing the relation between the cross-section of the wire 

 in the thermo-element and the temperature of the junction. This curve, 

 extended in the direction of decreasing cross-section, served to indicate 



Relative Cross-sections. 

 Figure 3. 



with at least a fair degree of accuracy the temperature which would 

 have been reached by a thermo-element of zero cross-section placed in 

 contact with the surface to be measured. The difference between this 

 temperature and that reached by a junction of any desired size gave the 

 correction which was to be applied. The correction, as will be seen by 

 inspection of the curve, Figure 3, is a very large one, amounting, even in 

 the case of the smallest wires which it was found practicable to use, to 



