132 PROCEEDINGS OF THK AMERICAN ACADEMY. 



closer adjustment of the spirals was made, as the method of experimen- 

 tation was expected to eliminate from the result any considerable error 

 arising from this inequality. That this expectation was finally justified 

 will be shown later, although more dilliculty was encountered than at 

 first appeared, the fact being, apparently, that the two spirals, although 

 made from the same piece of wire, did not have quite the same tempera- 

 ture coefficient of resistance, so that the difference of resistance between 

 them was not constant when their temperature was varied,* but increased 

 when the spirals were heated. 



Calibration of Differential Platinum Thermometer. 



In order to calibrate this differential platinum thermometer, the two 

 spirals were placed in streams of water, the temperatures of which were 

 measured by means of the same thermometers that were used in studying 

 the copper-iron thermo-electric junctions, and the difference between the 

 electric resistance of spiral No. 1, with its connecting wires, and that of 

 spiral No. 2, with its connecting wires, was measured by means of a 

 Carey Foster bridge. The conditions under which this trial was made 

 resembled very closely those under which the spirals were to be used in 

 the main experiment, the plugs bearing the spirals being inserted in the 

 same sockets in which they were afterward to be used, which sockets 

 were temporarily removed from the main apparatus and fitted to brass 

 tubes through which flowed the streams of water employed for the test ; 

 these tubes were wrapped around with cotton wadding, and sockets and 

 tubes together were surrounded by the same water-jacket that was used 

 later to surround the main apparatus. The copper connecting wires led 

 down between the cotton wadding and the water-jacket, and then out 

 beneath the latter without touching it ; this detail is mentioned because 

 tin temperature of the connecting wires is by no means a matter of in- 

 difference in some parts of the investigation. In this calibration test 

 each stream flowed past the thermometer bulb before reaching its spiral, 

 and past the spiral before reaching the ends of the copper wires which 

 carried the spiral. The distance from each thermometer bulb to the 



* The wire from which the spirals was made was annealed by drawing it through 



a flame, which treatment may have- introduced into it BOme luck of uniformity. 

 After the spirals were formed anil soldered to the copper they were kept in a bath 

 of melted parafflne, in the neighborhood of 146° <'., about two hours, with the 

 object of removing inequalities of condition caused by the bending to which the 



w ire had been subjected. 



