344 



E. L. NIOHOLS. 



a 



The inicrometer t^crew, by means of wliich tlie flaine is moved along the 

 bar^ is operated by means of a long handle with a universal joint, not 

 sliown in Pig. 5, so that the iiarne can be shifted by an observer sitting 

 opposite the ground glass screen. 



For the measnrement of tlie electromotive forces by the heating of 

 the junction a potentio- 

 ineter the arrangement 

 of which is showu in 

 (Pig. 7) is employed. It 

 consists of a sensitive 

 for m of d'Arsonval gal- 

 vanoraeter, (//) a résis- 

 tance box of 100/JOO 

 ohms (/r) and a standard 

 cell, (C). A slide wire 



R 



/ 



V 



/ V 



Fi,u'. 7. 



arrangement may be used in connection with this box in cases in 

 whicli it is desired to carry the bahmcing of the potentiometer beyond 

 the point reached by the insertion or removal of the 1 ohm coils, 

 but for most purjioses the adjustmeut is sufficiently accurate without 

 this addition to the apparatus. The electromotive force of the junction 

 J is balanced against that between the point {a) and the variable 

 contact point [h) in the usual manner. The metals tlie melting tem- 

 pératures of which are to form poiuts upon the calibration curve, 

 are worked into tliin foil and from this foil strips about .03 cm. in 

 width are eut. Such a strip is looped into tlie angle of the V and drawn 

 suugly into ])lace ; the free ends being eut away until they project only 

 about 1 mm. beyond the face of the junction. To hold this minute loop 

 of métal in its place, it is only necessary to press the foil carefuUy 

 together around the junction. The thermo-junction carrying the loop 

 having been mounted in the manner just described in the focus of the 

 caméra, will be clearly seen upon the ground glass screen, the ends of 

 the loop of métal projectiiig towards the flame. 



The détermination of the electromotive force corresponding to the 

 melting point is made as follows. The observer seats himself in a position 

 where he can watch closely the image of the flame and of the thermo- 

 element and moves the former gradually toward the junction, by means 

 of the long handle, balancing the potentiometer approximately from time 

 to time as the electromotive force rises with the increasing tem])erature. 



