. Bulletin . sof quartz tubing at each end (see Fig. 3). At the higher tem-peratures, however, the insulator softened and sagged down in themiddle, and two additional pieces of quartz were placed under it. eachone-third the distance from either end. The holes in the end of the fur-nace were packed Avith asbestos to prevent the escape of heat. Theheating coil was made a few turns longer than the test-piece to pre-vent cooling of the ends. IV. Method of Measurements The current was measured by a Weston direct-reading portablestandard millivoltmeter and shunt. The current was taken from 110-volt
. Bulletin . sof quartz tubing at each end (see Fig. 3). At the higher tem-peratures, however, the insulator softened and sagged down in themiddle, and two additional pieces of quartz were placed under it. eachone-third the distance from either end. The holes in the end of the fur-nace were packed Avith asbestos to prevent the escape of heat. Theheating coil was made a few turns longer than the test-piece to pre-vent cooling of the ends. IV. Method of Measurements The current was measured by a Weston direct-reading portablestandard millivoltmeter and shunt. The current was taken from 110-volt mains and was regulated by means of two large rheostats, as inthe calibration work, (Appendix). As it was necessary to maintainthe current constant to one-tenth of an ampere for several hoursat a time, and the line voltage varied considerably, a storage batterywas placed across the mains as shown in Fig. 4. By means of anotherresistance, the current from the mains was adjusted to a few amperes ..^w^^-^^2U. Fig. 4 less than that flowing through the furnace. In this way only a fewamperes were drawn from the battery and its voltage remained con-stant through a considerable change of the line voltage. This was notalways sufficient, however, for satisfactory operation. The voltage drop through the coil was measured by a Westonportable voltmeter, the smallest division of which corresponded toone volt. The temperature coefficient of nickel is very large, andthe resistance of the wire varied considerably with the the turns at the end of the coil were somewhat cooler than those 10 ILLINOIS ENGINEERING EXPERI:MENT STATION in the middle, their resistance was less, and the average voltage dropper centimeter for the whole coil would be less than the drop percentimeter of the central portion. As the average value was usedin the calculations, an error would thus be introduced. This wascompensated for, by having the potential leads connected to the heat-ing circuit, as state
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