78 PROCEEDINGS OF THE AMERICAN ACADEMY, 



The iatroduction of the thermo-element having been successfully car- 

 ried out by the method just described, it was possible to insert the plug, 

 carrying the rod and thermo-junction with it, into the end of the box 

 and to secure it in place ; after which the free terminal of the rod was 

 introduced between the jaws of a strong clip attached to the opposite 

 plug (b, Figure 1). This operation had to be performed through the open 

 windows in the side of the box. These were then screwed rightly into 

 place, and the box was ready for the exhaustion of the air. 



This method of measuring the temperature of the surface, to be suc- 

 cessful, involved the fulfilling of several rather difficult conditions and 

 the application of an important correction. To bore into the material of 

 a carbon rod carrying a current in the manner described, necessarily dis- 

 turbs more or less the flow of the current ; and the changes of resistance 

 thus introduced are likely to bring about decided changes of tempera- 

 ture in that neighborhood. In some instances this became obvious when 

 the rod was heated, the temperature being higher near the hole than else- 

 where. Indeed, it was often possible to note this effect with the eye on 

 account of the increased incandescence of the region in question. In all 

 such cases the mounting was rejected. It was found possible, however, 

 to so nearly compensate for this loss of carbon by the introduction of the 

 platinum junction that no difference in the incandescence of the surface 

 could be detected by the closest observation ; and since differences of 

 temperature which cannot be detected by the eye will be negligible in 

 spectrophotometric work, this was taken as the criterion of a satisfactory 

 mounting of the thermo-junction. Measurements were attempted only 

 when this condition was fulfilled. It is likewise obvious that there is 

 danger from the contact of the two wires of the thermo-junction with the 

 sides of the hole in the rod. A branch circuit for the passage of the 

 current is thus formed which includes the galvanometer coils, thus im- 

 perilling the integrity of the readings of the electromotive force. This 

 could be obviated only by having the wires touch the rod at points in an 

 equipotential surface, and the fulfilment of this condition was determined 

 by the reversal of the current through the rod and the absence of any 

 effect of such reversal upon the galvanometer. 



Another and more serious objection to the method, and one which 

 could only be met by the introduction of a correction, lay in the fact that 

 even with the smallest wires which could be used for a thermo-element a 

 certain amount of heat would be carried away by conduction through the 

 metal ; so that the junction would never reach the full temperature of 

 the surfaces with which it was in contact. I was at first inclined to think 



