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 reversing switch made as follows will be found very serviceable in reversing the direction of the rotation of small motors, changing the polarity of electromagnets, etc.
A diagram of the connections to the switch and on the switch base is given in the sketch, and in this particular case the switch is shown connected to a small toy motor. The field of the motor is represented by A, the armature by B ; and C, D, E, and F are four binding posts mounted on the base of the switch ; G, H, and I are three contacts ; J and K are terminals of the switch blades, and L a single-pole switch. The two blades of the reversing switch have their lower ends fastened to the terminals J and K, and their upper ends, which are indicated by arrow heads, may be moved over the contacts G, H, and I. For the position of the reversing switch shown by the full lines, J is connected to G and K to H. When the switch is thrown to the right-hand position, as shown by the dotted lines, J is connected to H and K to I. It is obvious that the direction of the current through the armature B will be reversed when the reversing switch is thrown from one position to the other. The direction in which the armature rotates will change, due to the reversal in direction of the current through it. The same results could be obtained by reversing the current in the field winding A. But it must always be borne in mind that in order to reverse the direction of rotation, the current must be reversed in the armature only or in the field only, not in both.
The above switch may be constructed as follows: First, procure a piece of well-seasoned hard wood, say maple, 1/2 in. thick, 2% in. wide and 4 in. long. Round off the corners and the edges of this piece on one side and drill the holes indicated in the sketch. The four corner holes should be of such a size as to accommodate the screws used in mounting four small back-connected binding posts. The remaining holes should be % in. All these holes should be countersunk with a %-in. bit to a depth of 1/4 in. on the under side.
Cut from some 1/16-in. sheet brass two pieces, 2% in. long, 1/2 in. wide at one end and 1/4 in. at the other, and round their ends. Drill a 1/8-in. hole through the larger end of each of these pieces, 1/4in. from the end, and also a hole through each, 1% in. from the narrow end. The last two holes should be threaded for 1/8-in. machine screws. Obtain five 1/8-in. brass bolts, 1/2 in- long. File the heads of three of these bolts down to a thickness of approximately 1/16 in. and mount them in the holes G, H and I. Before mounting anything on the base the grooves indicated by the heavy dotted lines should be cut in the under side so that the various points may be properly connected by conductors placed in the grooves. Now mount the two pieces of sheet brass upon the base by means of the remaining two bolts, which should pass through the holes J and K. A 1/16-in. washer should be placed between the pieces of brass and the wooden base.
Procure a piece of %-in. fiber, 1 1/4in. long and % in. broad. Drill two %-in. holes in this piece, one in each end, so that they are 1 in. apart. Drill a third %-in. hole in the center and fasten a small handle to the piece of fiber. Now mount this piece upon the two pieces of brass that form the blades of the switch by means of two small %-in. brass machine screws.
Two small brads should be driven into the wooden base so as to prevent the possibility of the switch blades moving beyond their proper position. Two pieces of 1/16-in. fiber should be placed between the heads of the screws G and H, and H and I, to prevent the ends of the switch blades from dropping down on the wooden base.
Ill: Diagram of the Wiring to a Small Motor and the Details of the Switch