106 A. I. Day and- It. B. Sosman — 



circumference of the bulb and the mean of their readings was 

 taken. 



In order to be perfectly certain that no systematic error 

 was being introduced by using this one form of furnace (fig. 2.) 

 throughout, it was replaced temporarily by a furnace of plati- 

 num wire wound on the outside of a similar tube. In this 

 way a heavy mass of good heat-conducting material was intro- 

 duced between the source of heat and the bulb, with the 

 expectation that a more uniform temperature might thereby 

 be obtained in the inside space. The two types of furnace 

 are shown in tigs. 2 and 3. 



A measurement at the copper point with the outside-wound 

 furnace gave as the melting point of copper 1082*6°, which 

 differs only 0*4° from 1082*2°, the mean of the results obtained 

 at the same pressure with the other furnace, and is identical 

 with the final mean of all the results, thus proving that no 

 systematic error was to be feared from the inside-wound type 

 of furnace. The horizontal uniformity obtained in the outside- 

 wound furnace was better than that in the inside-wound, but 

 the furnace was more difficult to regulate and to hold at a 

 given temperature. 



Q>) Constancy of Conditions. — Several causes interfered 

 with the establishment of a constant temperature for observa- 

 tion. The three heating currents required constant observa- 

 tion and readjustment with the gradual extension of the heated 

 zone toward the outside of the furnace. This comes to equi- 

 librium for a particular temperature after about half an hour, 

 after which the bulb was held steady 15-30 minutes longer 

 before readings of the pressure were taken. The temperature 

 thus established could be relied upon to remain constant to 

 within 1 to 3 microvolts (0*1° to 0*3°) during the course of the 

 pressure measurements. 



Above 1100° a noticeable leakage of current from the heat- 

 ing coil into the bulb and thermoelements frequently appeared. 

 This may have been due in part to conductivity across the 

 narrow air space between bulb and coil, but was probably 

 chiefly due to accidental contact between the protecting tube 

 of one of the thermoelements and the furnace wall. To obvi- 

 ate any uncertainty from this cause, it was found necessary to 

 use alternating current for all temperatures above 1100°. This 

 was less easy to regulate than the direct current from storage 

 batteries, but by careful regulation of the voltage of the motor 

 generator supplying the alternating current, equally satisfac- 

 tory results were obtained. 



The constancy and exactness of the temperature at 0° were 

 beyond question. On several occasions pressure measurements 

 at 0° were made at intervals of one-half to one hour and no 



