84 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Figure 7. 



size (diameter about 0.01 cm.), and a thermo-element is formed by cut- 

 ting pieces of the platinum wire, and of the wire of the alloy to be used, 

 about 70 cm. in length, and binding these to the opposite faces of a 

 rectangular block of wood about 1 cm. in thickness. Beyond this block 



the wires project about o cm. They are bent 

 toward each other until the free ends are in 

 contact, forming a V, and these ends are then 

 fused in the oxyhydrogen flame, forming a 

 junction, which is subsequently trimmed down 

 to the form shown in Figure 7. The apex of 

 the V is cut away until the arch of fused metal 

 joining the two wires is considerably less in 

 thickness than the diameter of the wires them- 

 selves, the face of the junction forming a smooth plane surface. 



The formation of such a junction becomes, with practice, a simple 

 matter, and can be performed, as it is necessary to do after each obser- 

 vation, in a few moments. The junction is rigidly mounted upon the 

 steel bar with the plane passing through the wires of the V vertical and 

 the plane surface of the metal which forms the face of the junction 

 parallel to the flat face of the acetylene flame. To the free ends of the 

 wires are soldered the copper terminals of the galvanometer circuit, and 

 the junctions are placed in a bath of meking ice. The support carrying 

 the thermo-element is mounted in such a position as to bring the face 

 of the hot junction as nearly as possible into the centre of the field of 

 view of the camera, where it is clearly visible under the illumination of 

 the acetylene flame, which should, at the beginning of the operation, be 

 about 1 cm. from the junction. The micrometer screw, by means of 

 which the flame is moved along the bar, is operated by means of a long 

 handle with a universal joint; so that the flame can be shifted by an 

 observer sitting opposite the ground-glass screen. For the measure- 

 ment of the electromotive forces produced by the heating of the junction 

 a potentiometer of the usual form is used. The metals the melting 

 temperatures of which are to form points upon the calibration curve, are 

 worked into thin foil, and from this foil strips about .03 cm. in width are 

 out. Such a strip is looped into the angle of the V and drawn snugly into 

 place, the free ends being cut away until they project only about 1 mm. 

 beyond the face of the junction. To hold this minute loop of metal 

 in its place, it is only necessary to press the foil carefully together 

 arouud the junction. The thermo-junction carrying the loop having 

 been mounted, in the manner described, in the focus of the camera, 



