This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A small electric furnace that will be very serviceable in a laboratory may be made as follows:
First procure a small clay flowerpot, about 4 in. in diameter at the bottom, and also a small clay crucible, about 2 in. in diameter at the bottom and at least 1 in. less in height than the flowerpot, and having as nearly as possible the same slope to its sides as the pot. Now obtain a small quantity of asbestos compound and pack it around the small crucible inside the flowerpot. Make sure the crucible is in the exact center of the flowerpot and that their tops are even with each other. Assuming that ordinary electric-light carbons are to be used, which are about 1/2in. in diameter, drill two5/8-in. holes, exactly opposite each other, through the walls of the flowerpot and asbestos compound so that they enter the crucible about 3/4 in. above its bottom on the inside. A suitable lid for the furnace may be made from 1/4-in. sheet asbestos and should be large enough to cover the top of the flowerpot.
The feeds and supports for the carbon electrodes are constructed as follows : Procure two pieces of 1/8-in. brass, 1 in. wide and 9 in. long. Cut a %-in. groove lengthwise in the center of these pieces to within 1 1/2in. of each end, as shown in Fig. 1. Drill four %-in. holes, AA, in each piece, a 3/8-in. hole, B, in one end and a 3/16-in.
hole, C, in the other end. Now bend the ends up at right angles to the remainder of the piece along the dotted lines shown at D and E. Next obtain two 3/8-in. rods, 10 3/16 in. long. Turn one end of each down to a 3/16-in. diameter for a distance of % in. From that point thread the same end of the rods for a distance of 6 in. Drill a 1/16-in. hole in each end of the rods a little less than 1/4 in. from the ends. The dimensions of the rods are given in Fig. 2. Two small rubber or wooden handles, similar to the one shown in Fig. 3, should now be made and fastened to the large ends of the rods by means of 1/16-in steel pins. Obtain two pieces of brass of approximately the following dimensions: 1 in. by 1 in. by 3 in. Drill four holes in each of these pieces as shown in Fig. 4. The hole H should be just large enough to allow the carbon to enter, or about 1/2 in.diameter.. The hole G should be tapped to take a 1/4 -in. machine screw, the hole F should be threaded so that the threaded rods will enter, and a small binding post should be mounted on a lug fastened in the hole J. Cut away one end of this piece as shown in Fig. 4 until it is a little less than 1/8 in. in thickness, or so it will enter the grooves cut in the brass strips.
Ill: Details of the Base, Rod and Handle for Each Carbon Feed, Which is Attached to the Large Base on One Side of the Furnace
The parts of the furnace are now ready to assemble, which may be done as follows: Procure a piece of well seasoned board, hard wood if possible, about 1 in. thick, 8 in. wide and perhaps 20 in. long. Cover one side of this board and the edges with some 1/8-in. sheet asbestos. Now place the flowerpot in the exact center and then mount the grooved brass strips one on either side of it with the longest dimension parallel to the longest dimension of the board and the inside end about 1 in. from the side of the pot. The end with the 3/16-in.holes should be next to the pot. Assemble the parts of the carbon feeds and then cut out some circular disks of asbestos to place under the flowerpot so as to raise it to such a position that the holes in its sides will be on a line with the carbon rods. Three long screws should now be placed in the board, forming the base, in such a position as to hold the flowerpot always in place. This completes the furnace proper, which is shown in Fig. 5. The furnace can now be put into operation provided there is a suitable current rheostat to connect in series with the carbon arc to prevent an excessive current being taken from the line. If such a rheostat is not available, a serviceable one may be made as follows:
Obtain two pieces of 1/16-in. sheet iron, 6 by 6 in., that are to form the end plates. Cut off the corners of one piece so as to form an octagon and drill a number of 1/8-in. and 1/4-in. holes in it, as shown in Fig. 6. Bend the corners of the other piece down along the dotted lines marked L, Fig. 7, and then make a second bend in each corner along the dotted lines K, so that the outermost portion of the corner is parallel to the main portion of the piece. Drill a number of 1/8-in. holes in this piece as indicated. A 3-in. opening should be cut in the center of this piece to give access to the interior of the completed rheostat. Now obtain eight 3/8-in. iron rods, 10 in. long. Drill and tap each end of these rods to accommodate a 1/8-in. machine screw. Wrap several layers of thin sheet asbestos around each rod and tie it in place with some thread. These rods should now be fastened between the end plates by means of a number of iron machine screws. Mount four back-connected binding posts on the plate shown in Fig. 6, making sure they are insulated from the plate by means of suitable bushings and washers.
Procure a small quantity of No. 14 gauge iron wire. Fasten one end of the wire under the head of the screw holding one of the binding posts in place and then wind it around the rods about 20 times, making the distance between the turns equal to the diameter of the wire. After winding on the 20 turns, attach a short piece of wire to the main wire and fasten the free end of the short piece to one of the other binding posts. Wind on 20 more turns, and make another connection to the third binding post, then complete the
Ill: Detail Of The Upper And Lower End Plates That Are Used In Making A Rheostat
The Furnace Consists Of A Flowerpot In 'Which A Crucible Is Set, And On Either Side The Carbon Holders Are Fastened To The Base winding and attach the end to the remaining- binding post. Different amounts of this resistance can now be connected in series with the arc by changing the connections from one binding post To another. The rheostat may be located on a bracket fastened to the wall, but care must be taken not to place it in such a position that it will come into contact with inflammable material. As an extra precaution, the circuit should be properly fused.