. Carnegie Institution of Washington publication. 26 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. Heating coil -,$. Fig. 3.—The furnace, showing ther- moelement and charge. The coil, which was obtained from Dr. Heraeus, was of platin- iridium wire (90 parts Pt., 10 parts Ir.), mm. in diameter, and required about 3000 watts to maintain a constant temperature of 16000 C. The furnace was carried at times on a 110-volt direct-current street main, but accurately constant temperatures could not be depended on without the storage battery. The insulation in these furnaces was so perfect that s
. Carnegie Institution of Washington publication. 26 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. Heating coil -,$. Fig. 3.—The furnace, showing ther- moelement and charge. The coil, which was obtained from Dr. Heraeus, was of platin- iridium wire (90 parts Pt., 10 parts Ir.), mm. in diameter, and required about 3000 watts to maintain a constant temperature of 16000 C. The furnace was carried at times on a 110-volt direct-current street main, but accurately constant temperatures could not be depended on without the storage battery. The insulation in these furnaces was so perfect that shutting off or reversing the heating current at the highest temperatures did not produce a quiver in the galvanom- eter to which the thermo-element was connected, although the sen- sitiveness of the system was such that a leakage amounting to a single micro-volt (corresponding to less than °) at 16000 would have caused a displacement of more than two millimeters on the scale. STANDARDS. The thermo-electrical potential was measured upon a potentiom- eter (Wolff, Berlin, Reichsanstalt calibration) in terms of a standard cadmium cell (saturated) prepared by ourselves. Two of these cells were used interchangeably during the earlier measurements. Toward the close of the series four fresh cells were prepared for com- parison with the earlier ones and were found to agree with them within One of these later cells (the readings of the four were iden- tical to the fifth significant figure) was verified bv Dr. Wolff, of the Bureau of Standards, by comparison with the standard Clark cells of that institution and found to be V at 200 C, assuming the legal value (United States) of the Clark cell, V, at 150 C. Substitut- ing the Reichsanstalt value, Clarke = ,* our cells would give a normal potential difference of at 200. The temperature determinations which follow are, therefore, calculated in terms of this number. With the apparatus here described, th
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