ON A NKW MKTIIOn Ol' CALl lîl{ ATING, &f;. 345 



At a dertnite distance from tlic Imiiiiious ciivelojje of tlie tlaine, wliic.li 

 distance dépends upon tlie character of the métal under investigation, 

 tlie projecting ends of the loop will be seen to melt. So qniet is the 

 Hame, and so well fixed the température gradient from its surface 

 outward when a proper bnrner is nsed, and when the flame is placed in 

 a localitj reasonably free from air curi'ents, that the fusion of the 

 successive portions of the métal loop may be brought about from the 

 end inward with the gveatest nicety; and the electromotive forces may 

 be determined at each stage until the fusion lias progressed to the plane 

 coinciding with the face of the junctiou. Even then in many cases, those 

 portions of the loop of métal which lie witliin the angle of the junction 

 will remaiu unfused althoiigh their distance from the melted portion of 

 the loop is oïdy a fraction of a millimeter. 



The delicacy of tliis o])eration under favorable conditions is very 

 great and th^ agreement of the successive readings of the melting point 

 of a giveii sample of métal is excellent. Tt is désirable to make a séries 

 of readings, leading up to the true melting point for the reason tliat 

 when the fusion of the métal loop has progressed to that portion which 

 lies in contact with the platinum, an alloy is alniost innnediately formed 

 between the fused métal aiul the junction itself, which affects the tliermo 

 electric indications of the couple, l^or this reason it is not possible to 

 get consistent readings by repeating observations with a given junction. 

 The proper procédure is to eut the wires back 2 or 3 mm. from the 

 a2)ex of the 1 after each set of readings and to make a new junction of 

 the proper form from the free ends thus produced. This recpiires but 

 little time after the operator has gained a reasonable degree of familiarity 

 with the method. 



When the métal, the melting point of which is desired, is platinum 

 itself, the platinum wire of the junction begins to fuse at the same time 

 as the loop, the platinum rhodium or platinum-iridium side remnining 

 urimelted. The pre;cise point at which this fusion of the platinum oc- 

 curs, is, hovA'ever, quite as definite as in the case of metals of lower mel- 

 ting température. In calibrating thermoelemeuts constructed from the 

 platinum and platinum-rhodium obtained from Heraus in Hanau from 

 the same stock as that used by Holboun and Wien ') in their well 



') Hoi.BoiiN and Wien. 1. o. 



