The electric furnace is a great favorite with experimentalists, in consequence of the very high degree of heat which is readily obtainable by its use, reaching from 3500° to 1000° Cent. (7232° Fahr.), while with the ordinary blast furnace 1800° C. is rarely surpassed. With the oxy-hydrogen blowpipe, a temperature of 2200° C= about 4000 F. has been obtained. It is not at all difficult either to make or to use such a furnace; and we purpose here, in the interest of such of our readers who make chemical or metallurgical experiments their hobby, to describe the construction of a small laboratory furnace, that can be actuated either from a battery of 25 bichromate or from any electric light main capable of giving a continuous current of from 8 to 10 amperes at 50 volts pressure.
To make such a furnace, we shall begin by procuring or sewing up, a slab of slate, about 1 in. thick by 8 in. square. If not already smooth, this can be levelled by rubbing it over a flat stone surface along with fine sand and water. For the crucible, in which our melting or electrolytic operations are to be effected, we shall do well to select one capable of containing about 2 pounds of ore, and constructed almost entirely of carbon. As carbon is a very fair conductor of elec-
tricity, there is no necessity for using a separate negative electrode, the crucible itself serving that purpose. Besides, it is practically infusible, itself highly refractory, and does not contain any matter that would oxidize or otherwise contaminate the results of our operations. To mount this crucible in such a manner as to make a good electrical contact with the terminal that will afterwards be used as the negative of our furnace, we procure a sheet of No. 22 gauge copper about 10 in. long and 4 in. wide. On its surface, concentric to one another, we inscribe, with a pair of compasses, two segments of circles, one of 11 in. and the other 9 in. in diameter and 1 in. apart, as shown in Fig. 1.
We then cut off the superfluous metal, indicated by the dotted lines, and bend the segment, shown by the heavy black lines, into the shape of an inverted cone, which should be made to fit exactly around the bottom of the crucible; the ends overlapping by about 1 in. A mark should now be made at the center of the overlapping portions, a hole put through both, and the conical ring thus formed, joined together by riveting through this hole with a flat-headed copper rivet, carefully hammered smooth.
From a strip of hard brass ribbon, about No. 30 gauge and 1/2 in. wide, we cut three lengths, one being 9 in. long, the other two only 7 in. We bend all three into the shape of a letter L, the two latter having the horizontal portion only 2 in. long, while the former has
its horizontal portion 3 in. long. These we rivet at three equi-distant points on the circumference of our conical ring, so that the ring stands 1 in. from the shorter limbs, which are then bent more acutely, so as to allow the tripod thus formed to stand quite level on its three feet. See Fig. 2. A 3-16 in. hole is now put through the center of each of these feet, the tripod with the crucible firmly pressed down in it, is placed centrally on the slate slab, with the longer foot pointing to one of the corners thereof.
At the points where the holes have been made through the feet, marks are made on the slate slab, the tripod removed, and three holes, countersunk underneath, are put through the slate at these points, into which three small metal screws are inserted from below, these screws being fitted with nuts to hold the feet in position. From a strip of thin brass 1/2 in. wide a perfectly cylindrical ring, 3 1/2 in. diameter, is now made by riveting the extremities together. This ring is used to draw the three prongs of the tripod towards one another, when the crucible has been placed between them, and this conduces to produce good electrical contact between the tripod and crucible. It is needless to say that the prongs of the tripod must be curved inwards, so as to follow as nearly as possible the curvature of the crucible.
A hole should now be made near the extreme end of the longer foot of the tripod, extending through the slate base, and here should be inserted and screwed in the shank of a fairly stout terminal fitted with a nut below. In the opposite corner of the slate we now drill a full 1/2 in. hole and countersink it underneath. We now take a round iron rod, 1/2 in. in diameter, and file, or better, turn down 1 1/8 in. at one extremity, on which we put 1/4 thread by the aid of a screw cutting die. Inserting temporarily the screwed end of this rod into the hole just made, we make a mark on it, at such a height as will clear the mouth of the crucible and its clamping ring by about 5 in.
We then red-hot the iron rod and bend it neatly and squarely at right angles at this point, and afterwards cut off at the bent end, whatever projects over 1/4 in. of the center of the crucible. At this extremity of the rod, exactly in line with the center of the crucible we drill and tap to 1/4 in. Whitworth, a hole, in which we fit a screwed brass rod about 9 in. long, bent twice at right angles, so as to form a handle to the screw itself.
We now take a piece of solid drawn brass tube, 2 in. long 3-16 in diameter inside, and with a 1/4 in. tap, put in it a 1/4i in. female thread for a depth of about 1/4 in., so as to grip firmly the end of the screwed brass rod. With a fine hack saw we split the tube in four quarters for a length of about 2 in., at the opposite end and fit this with an outer ring, which should be a sufficiently tight fit to bring the sides of the split ends together.
This tube serves as the carbon holder, the carbons to be used with it being known 'as the " 6 millimeter," or 1 in. size.
These carbon rods should be about 4 in. long and at least 2 in. should be inserted into the tube or holder. An oval brass washer, 1 1/2 in. long by 1 in. wide, with a i in. hole at one extremity, is now put over the 1 1/4 in. screwed end of the bent iron rod ; this is inserted in its hole in the slate base, care of course being taken that the center of this iron rod and its carbon holder coincide with the center of the crucible. Lastly, we drill a hole through the extended portion of the brass oval washer, and partly into the slate base and fit therein a second terminal similar to the one at the opposite corner. To use this furnace, which we represent in section at Fig. 3, and in perspective at Fig. 4, it is only necessary to connect up the positive pole of the battery or dynamo to the terminal T+, the negative to T - , and lower the carbon by means of the screw S, until the arch has been struck between the carbon rod and the bottom of the crucible, immediately pouring into the crucible a little of the material to be operated upon, being careful to maintain the arc, by duly lowering or raising the carbon rod, and adding more material in proportion as it becomes fused orelectrolysed. -"Hobbies," London.