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virtually the same structure as the stable crystalline form. However, there can be little doubt of its higher energy content, for the heat of hydration mas determined by two entirely different calorimetric methods on three separate preparations. The unstable, highly reactive variety obtained by dissociation in a vacuum at 100" C. is probably largely a mixture of undissociated gypsum and soluble anhydrite, since its heat of hydration of about - 5200 calories approximates the -5400 calories that would be expected from such a mixture. The heat of hydration of strictly anhydrous soluble anhydrite could not be determined because it was impossible to prepare such material. Heating for days a t 100" C. in a vacuum corresponding to l o p 4mm. of mercury left the material with a constant water content of around 0.2 per cent. Temperatures in excess of 200" C. were required for complete dehydration. Under these conditions partial conversion to insoluble anhydrite always occurred. To estimate the heat of hydration of absolutely anhydrous soluble anhydrite, it was necessary to extrapolate the values obtained on samples containing various amounts of water. il plot of the heat of hydration us. moles water per mole of calcium sulfate is shown in Figure 2. A straight line drawn through these points closely represents the data of all the hydrates prepared in a gaseous medium below 100" C. The intersection of this line at the zero water-content axis gives -7210 * 10 calories per g. f . w.as the heat of hydration of soluble anhydrite. From this same line the heat of hydration of the metastable hemihydrate was taken to be -4600 calories. The heats of hydration of natural anhydrite and of insoluble anhydrite prepared a t 870" t o 900" C. were found to be -4040 * 20 and -4030 + 20 calories per g. f. w.,respectively. The virtual coincidence of these heats indicates that complete crystallization of anhydrite from the soluble to insoluble form had taken place. The heat of hydration of soluble anhydrite was estimated
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by Sewman and Wells (9) as being not less than 6990; they state that the heat of transition of soluble anhydrite to natural anhydrite is not less than 3000 calories per g. f. TV. These values are not a t variance with the more definite ones determined in this investigation. Heats of solution in hydrochloric acid were also made on three forms of CaS04.2Hz0-namely, selenite, microcrystalline gypsum, and set plaster. The values obtained for the first two are virtually identical; that of set plaster does not vary from the others by more than can be accounted for by uncertainties in analysis and the corrections involved. It is therefore concluded that there is little if any difference in energy content in the various varieties of CaS04.2HL0.
Acknowledgment The helpful advice and encouragement of C. G. hfaier and K. K. Kelley of the U. S. Bureau of Mines and of W.C. Riddell of the Pacific Portland Cement Company is gratefully acknowledged.
Literature Cited (1) BHchstrom, H. L., J . Am. Chem. SOC.,47,2432 (1925). (2) Bussem, UT., and Gallitelli, P., 2.Krist., 96, 376 (1937). (3) Caspari, W.A., Proc. Roy. SOC.(London), A155,41 (1936). (4) Feitknecht, W., Helv. Chim. Acta, 14,85 (1931). (5) Gallitelli, P., Periodico mineral. (Rome), 4, 159 (1933). (6) Hoff, J. H. van't, Armstrong, E. F., Hinrichsen, W., Weigert, F., and Just, G., 2. physilz. Chem., 45,257 (1903). (7) Maier, C. G., J . Am. Chem. SOC.,52,2160 (1930). (8) Maier, C. G., J . Phys. Chem., 34, 2866 (1930). (9) Kewman, E. S., and Wells, L. S., J . Research Natl. Bur. Standards, 20, 825 (1938). (10) Onorato, E., Periodico mineral. (Rome), 3, 135 (1932). (11) Riddell, W. C., private communication. (12) Rossini, F. R . , J . Research .\-atatl. Bur. Standards, 9, 697 (1932). PVBLISHED by permission of the Director, Bureau of Mines, United States Department of the Interior. ( N o t subject t o copyright.)
Overcoming Thermocouple Errors in HighTemperature Induction Furnaces FRANK DAY, JR., AND HURD W. SAFFORD University of P i t t s b u r g h , P i t t s b u r g h , Penna.
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FIGURE 1
HIS paper is published so that individuals employing thermocouples in graphite-core tube-type induction furnaces may obtain constant readings. The furnace employed contained a copper-tube induction coil. JThen a thermocouple (22-gage platinum-platinum, 10 per cent rhodium) was placed in the porcelain combustion tube in which experiments are made and the temperature recorded automatically through a potentiometer, the curve was regular up to about 1300" F. and then became erratic (Figure 1, curve A ) . That this effect was obtained only while the induced current was operating was obvious, for when the latter was discontinued the curve again became regular. The irregularity is due to an electromotive force which is set up in the thermocouple in opposition to that normally developed. This might possibly be attributed to thermionic emission from the graphite core a t relatively high temperatures. The emission may build up a charge on the protection tube covering the thermocouple and the porcelain may become sufficiently conducting to carry the charge to the interior.
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To overcome the difficulty, the authors enclosed the protecting tube of the thermocouple with platinum foil t o the depth of immersion in the furnace. This raised the temperature of “irregular” recording about 200’ F. Attaching a wire to the shield and grounding it completely eliminated the difficulty. Since the exposure of the platinum shield to fumes in the furnace was considered undesirable, a shield of smaller diameter was placed over the insulating tube inside the porcelain protector of the thermocouple and again grounded as previously stated. This modification also requires less of the costly platinum and prevents its abrasion. It is necessary only to have the platinum shield cover that portion of the pyrometer which is actually in the furnace.
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It had already been observed, when the curve became irregular with the induced current operating and this was shut off, that a regular curve resulted. The curves obtained with the grounded platinum shield correspond to the latter. Records obtained with the protected thermocouple are indicated in curve B , Figure 1. The remedy suggested should make it possible to use thermocouples for induction tube furnaces containing no metallic conductors other than the thermocouple. It may find application in other types of induction furnaces. CONTRIBUTION 377 from the Department of Chemistry, University of Pitteburgh.
Electrical Phenomena in a High-Temperature Laboratory Furnace H. E. STAUSS Baker & Co., Inc., Newark, N. J.
HE preceding note of Day and Safford illustrates the difficulties encountered in the use of high temperatures; phenomena occur which, while not new, are not always anticipated. In this laboratory we have been determining the electrical resistance of some platinum alloys a t high temperatures. The test wire is wound on an alundum core and placed in an alundum tube furnace using a platinum alloy wire as resistor. As the furnace temperature reaches about 1200’ C. (2190” F.) the galvanometer in the Wheatstone bridge circuit no longer deflects smoothly, but “kicks” erratically as if an induced current is present. When the cell is disconnected from the circuit, the galvanometer assumes a fairly steady deflection while its key is pressed. These direct currents were observed with alternating current in the furnace. The two ends of the resistance coil showed a potential difference of 0.3 millivolt d. c., with 100 volts a. c. in the furnace. When a platinum-platinum 10 per cent rhodium thermocouple in an alundum tube and a bare platinum wire were placed in the furnace from opposite ends so as not to touch each other or the furnace, d. C. potential differences (measuring voltages instead of currents) were obtained between the wire and either leg of the couple. The values obtained with direct current in the furnace resistor were 3 and 7 millivolts, the sign and magnitude of the reading
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changing with the direction of the current in furnace. The value dropped to 0.8 millivolt with alternating current in the furnace. With two bare platinum wires in the furnace as electrodes, the potential difference was only 0.2 millivolt, but was sensitive to position and increased to 1.5 millivolts when the wires were parallel and overlapped. Thus it appeared certain that thermionic emission was occurring in the furnace, with the alundum as an active emitter. In addition, the investigation of the phenomenon revealed the existence of a potential drop between the resistance coil under test and ground. At 1400’ C. (2550” F.) this had a value of about 5 volts, with the furnace resistor a t 132 volts a. c. The conductivity of the alundum had become sufficiently great to be an important factor in the use of the furnace. Two adjacent turns of wire embedded in alundum short-circuited when the potential difference between them was raised to about 100 volts. The best practical solution for the elimination of the stray currents, and one which should be a minimum safeguard for electrical measurements a t high temperatures, was found to be in the insertion between the furnace tube and the resistance coil of a grounded shield, connected to the coil and to the ground side of the furnace winding. Platinum wire wound on a ceramic tube functioned very well as the protecting shield in our experiments.
