NICHOLS. — THE VISIBLE RADIATION FROM CARBON 



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to locate, by a variety of methods, the hot junction of the thermo-element, 

 by means of which the temperature of the surface of the roils was to be 

 measured, in such manner that it would assume the temperature of that 

 surface, made it only too clear that herein lay one of the chief difficulties 

 of the investigation. It was found that such a junction, however small 

 its size, and however carefully it might be brought into contact with the 

 surface of the rod, would not take even approximately the temperature of 

 that surface ; and recourse, after the failure of numerous other expedients, 

 was had to the following plan, which although far from being free from 

 objection, was found to be upon the whole the most reliable, and to give, 

 when properly carried out, the most definite and satisfactory result. 



By means of a drill made for the purpose from the smallest obtainable 

 size of steel sewing-needle, a minute hole was bored radially at a point 

 upon the surface of the rod lying within the field of view of the spectro- 

 photometer. This hole had an approximate diameter of 0.03 cm. It 

 extended to a depth equal to about one half the radius of the rod and was 

 conical in form. Platinum and platinum-rhodium wires to he used for 

 the thermo-element were drawn to a diameter of 0.0 1G cm., and their 

 free ends having been. laid together side by side, were fused in the flame 

 of the oxyhydrogen blowpipe so as to form a junction. This junction, 

 which after the action of the blowpipe took the shape of a small bead of 

 the combined metals, was trimmed down into conical form, until it would 

 just enter the hole in the side of the rod, care being taken that the 

 entire junction was beneath the surface. The wires leading from this 

 junction were next sealed into a glass tube of about 2 mm. bore, through 

 the interior of which they were carried from end to end, care being 

 taken that they should be nowhere in contact. They were held in place 

 by fusing the glass around them at either end of the tube. This tube 

 was inserted through an opening in the plug a 

 (Figure 1) carrying one terminal of the rod, 

 and there made air tight by means of cement. 

 One end of the carbon rod was then inserted 

 in a clamp attached to the inner face of the 

 plu"-, and the wires at a distance of about 

 1 cm. from the junction were bent downward 

 at right-angles, so as to bring the junction 

 into position for insertion into the hole in the 

 rod, and to hold it there when inserted by 

 the slight but sufficient spring-action of the 



wires themselves. This arrangement of the junction and rod is indicated 

 in Figure 2. 



Figure 2. 



