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The Chemistry Student
SOME FAILURES AND SUCCESSES WITH HIGH FREQUENCY FURNACES JOHNKISSBL AND WILLIAMHAMM,MARYHURST NORMAL.KIRKWOOD, MISSOURI I n the November, 1928, issue of THISJOURNAL (pp. 1491-2) there appeared an article describing the construction of a small arc furnace. At the end of the article, reference was made to an induction furnace which the author threatened to tell about "if dreams came true." Well, they have come true, and this article bears out the threat. We read what literature we could find on the subject of high frequency furnaces, and that was very little. However, we were impressed with the
THEHIGH FRBQUENCY OvTllrT ASSEMBLED T, transformer; C, condenser; S, spark gap; F, furnace; G,another furnace not connected.
speed with which these furnaces work, and the ease and exactitude with which the conditions of an experiment can be reproduced. We therefore concluded that such a furnace would certainly be a very desirable piece of laboratory apparatus, and then set about to construct one. We first constructed a high frequency apparatus of one-half kilowatt capacity. This was designed primarily for a Tesla coil, but we hoped that it would operate a furnace also. The primary coil of the transformer con-
sisted of about 325 turns of No. 14 wire and the secondary of 16,500 turns of No. 30 wire. These coils were wound by hand on an improvised winder, and the operation consumed all our free time during several weeks. The condenser was made of 120 5" X 7" negatives from which the emulsion had been removed. Sheets of tinfoil were placed between the plates and properly connected. The assembled condedser was then immersed in an insulating mixture of melted beeswax and rosin. The spark gap was of the stationary kind formerly used in wireless sets. The first furnace was a very rudimentary affair. It consisted merely of a helix of heavy copper wire wound around a glass tumbler. We put a crucible full of type metal into the tumbler, made the connections, and turned on the current. In a few minutes the type metal became hot, and soon a part of i t melted. That was encouraging. We now set about making what we considered to be a real furnace. We
CONDEN~E~
F ' V ~ A C EJND~TXNCE DIAGRAM oa FURNACE HOOK-UP
wound a twenty-five foot length of 3/,a-inch copper tubing into a helix about four inches in diameter; then, by a laborious and unpatented method, flattened the tubing so as to increase the number of turns per inch without, however, damaging the tubing to such an extent that i t could not again be used to transmit a flow of water. When completed, the helix measured a little more than four inches in height and had about twenty-eight turns. Retort cement was used to insulate the turns from each other, and a liberal coating on the inside of the coil made it quite rigid. A clay crucible was then placed inside the coil and magnesite packed tightly around it. We placed an ingot of aluminum in the crucible and turned on the current. Now, we had agreed that the aluminum must be melted before the furnace could be called a success. After a ten-minute run we uncovered the crucible, only to find the aluminum about as cold as when we had put it in. Several subsequent runs, made after all the connections, etc., had been gone over, gave the same result. Evidently, our furnace was a failure.
VOL.6, No. 11
THEC ~ M I SSTUDENT ~ Y
2001
We made another by cutting rings out of sheet copper, cutting them open, and soldering a number of them together to form a helix. This gave a great number of turns per inch, but the coils got hotter than the contents of the crucible. Then our big furnace was dismantled and reduced in size to about 2'/%inches in diameter, but without satisfactory result. We succeeded in melting zinc, but zinc is not aluminum. We were forced to conclude that our generator was not sufficiently powerful. So it was up to us to construct a more powerful generator, or quit. Now, quitting is not our long suit. We disposed of the 1/2-kilowattoutfit and procured the material for a 5-kilowatt transformer. Profiting by previous experience, we soon had the coils wound and the transformer assembled. This time the primary had only 100 turns of No. 7 wire and the secondary about 3600 of No. 24. The condenser was constructed along the same lines as the one previously used, but it was much larger. We used 8" X 10" plates and immersed the condenser in oil instead of the messy beeswax-rosin mixture. We also fabricated a rotary spark gap from material available in the physical laboratory. The furnace was once more dismantled and remodeled, its diameter being reduced to a little more than two inches. A very serviceable crucible was made by boring a 5/s-inchhole in a battery carbon to a depth of about three inches. This crucible was again insulated from the coil with magnesite. All parts being properly assembled and connected, we were ready for the test. We began gently by filling the crucible with type metal. It melted in ten seconds. That was more than encouraging, and we immediately put into the crucible our famous ingot of aluminum. In one minute i t had turned into a liquid. That meant success according to our definition, but we had become ambitious and kept right on. Brass melted in five minutes, copper in eight, and iron in a little more than twelve. Now we are ready to pronounce our furnace a success according to any definition. These experiments consumed the greater part of our spare time for nearly a year, yet we enjoyed the work immensely and found it extremely inter. esting. And now, to be sure, we know considerably more about high frequency furnaces than we did when we began.
