436 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Two voltmeters with scales from 0-65 and 40 to 160 were used. These 

 were calibrated so as to make them comparable with each other. The 

 alternating current instruments were of the Thomson type made by 

 the General Electric Company. 



In calibrating the furnace the wires of the pyrometer were protected 

 by fused silica tubes which extended up into the tower in the lid of the 

 furnace. The tubes were covered at the junction by a short graphite 

 tube. This projected through a hole in the cap of the crucible con- 

 taining the charge and rested in the charge. The bare wires were 

 brought out of the furnace at the top of the tower between rubber 

 washers ; the furnace was then evacuated and the calibration taken. 



TABLE III. 

 Variation of Temperature in Crucible. 



The power was 8.36 kilowatts. 



The carbon shield surrounding the spiral was not used in these ex- 

 periments on account of the fact that carbon absorbs a large amount of 

 gas which is not easily removed. It will be evident from the method 

 of experimenting described below that its use would not be permissible. 



In the figure a circle is put around those points taken with the first 

 spiral. It is evident from this figure that this method of obtaining the 

 temperature is not as accurate as the Wanner pyrometer would be 

 were it in good condition. 



It will be seen that there is no regular difference in the calibration 

 of the two spirals, except that all the points of the first coil lie on the 

 upper dotted line, while some of the points for the second coil lie on 

 the upper as well as the lower. This is probably due to the fact that 

 the second spiral was calibrated more than once. It was thought best 

 under the circumstances to draw the solid line midway between the 

 two extremes and take this for estimating the temperature. 



A further test was made to see how constant the temperature was 

 throughout the length of the crucible. For this purpose the junction, 



