224 PROCEEDINGS OF THE AMERICAN ACADEMY. 



beyond doubt. The phenomenon was that after a few minutes of 

 constant temperature at the melting point the indication of the thermo- 

 couple fell off with increasing rapidity. On withdrawing the couple, 

 cleaning it, or clipping it off and restoring it to jilace, the melting 

 metal meanwhile being untouched, the indications returned to their 

 original high value. The apparent explanation was the formation of 

 a slag between the wires, but this was not entirely satisfactory. The 

 use of a plumbago crucible in place of the clay, and an entirely fresh 

 lot of aluminum, did not remove the phenomenon, and gave the same 

 initial readings, which it could not be doubted were the ones corre- 

 sponding to the melting point. The fusion of the aluminum was, how- 

 ever, the least sharply defined of all the metals used. 



The fusion of platinum was, of course, differently effected. For 

 this the two wires of the couple were laid close together on a piece of 

 lime. An oxy-hydrogen flame was then directed upon their ends, and 

 the platinum fused into a globule which with care could be made to 

 travel slowly up the wire. There was no difficulty in ol)taining 

 steady temperatui"es for a sufficient period to make the necessary read- 

 ings, and check results to 0.1 per cent were obtained on different 

 days. 



The galvanometer, keys, coils, and all junctions of dissimilar metals, 

 were, so far as possible, covered with boxes of wood, pasteboard, or 

 asbestos, to maintain uniformity of temperature, and thus minimize 

 local thermo-electric disturbances. With this precaution no sensible 

 trouble from that source was experienced. 



The procedure is as follows. To take the observation for vapor 

 of sulphur, for instance, the hot and cold junctions are exposed as 

 described. After a sufficient time the main circuit is closed, the ther- 

 mal circuit is connected to a suitable part of a, b, c, d, and the rheo- 

 stats, W, are adjusted until on pressing the key no deflection occurs 

 in the galvanometer, G. At this instant, A is read, which gives the 

 current c in the main circuit. The adjustment is disturbed and re- 

 made a number of times, and the resulting readings should check to 

 the nearest tenth of a division of A, provided the metal has reached 

 a steady state of temperature. 



By this adjustment the drop of potential c r due to the current c 

 amperes in the part r ohms of the resistance a, h, c, <f, spanned by 

 the thermal circuit, is made equal to the total resultant eraf. in the 

 thermal circuit. The latter, which will be denoted by 2^ e or S^ ^j is 

 the algebraic sum of the thermal emf. proper of the junctions of the 

 Thomson emf. in the wires, and of any " stray " or local thermal emf. 



