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.
Procure two pieces of metal, one of brass and the other of sheet iron, 5 in. long, 1/2 in. wide, and 1/32 in., or just a little more, in thickness. Bend the brass strip into the form shown in Fig. 1, then place the brass piece on top of the iron and drill the holes A and B indicated in Fig. 2. After the brass piece has been bent, as shown in Fig. 1, it will of course be shorter than the iron strip and the iron strip must be cut off, or a brass strip a little longer than 5 in. can be secured and cut the same length as the iron strip after it is bent. The holes A and B should be 3/32 in. in diameter. The next thing to do will be to wind a heating coil about the brass strip. Wrap a very thin layer of sheet asbestos about the brass strip, and wind on the strip 18 ft. of No. 34 gauge bare superior resistance wire. Use a thread about .006 in. in diameter to separate the various turns. This thread can be removed after the winding is completed and the ends have been fastened. Rivet the iron and brass pieces together with a small brass rivet in the hole A, Fig. 2. After the two pieces are riveted together bend them into the form shown in Fig. 4 and then drill the two 1/8-in. holes C and D, as shown in Fig. 2. Tap the hole B, Fig. 2, to take a small machine screw.
The base is constructed as follows: Procure a piece of slate, 5% in. long, 1 1/2 in. wide, and 1/2 in. in thickness. Drill the holes indicated in Fig. 3. The four corner holes are for mount-
Ill: Dimensions of the Brass Strip and Mounting Base, Showing the Location of the Holes and the Shape of the
Brass Strip to Receive the Coil of Wire ing the flasher in its containing case, and should be about 1/8 in. in diameter. The holes E, F, G, and H should be 1/8 in. in diameter and countersunk with a 3/8-in. square-ended drill, on the under side, to a depth of 3/16 or 1/4 in. Cut from some 1/16-in. sheet brass a piece 1% in. long, and 1/2 in. wide. Drill two 1/8-in. holes in this piece, 7/8 in. apart and equally spaced from the ends. Procure four 1/8 -in. brass bolts, two 1/2 in. in length, and two 1 in. in length. Secure four small washers and two additional nuts. Mount the combined iron and brass strip on the slate base, using a long and short bolt as shown in Fig. 4. One terminal of the winding should be placed under the head of the bolt J. Place a washer, K, between the head of the bolt and the wire. The brass strip L can now be mounted in a similar manner, as shown in Fig. 4. Place the other end of the winding under the head of the bolt M.
Obtain a small screw, N, Fig. 4, of such a length that its point will reach the brass strip L when the screw is placed in the hole B, Fig. 2. A lock nut, O, should be provided for this screw so that it will remain in adjustment. The point of the screw and the point on the brass plate where the screw touches should be of platinum, as the brass will not withstand the high temperature of the arc formed when the circuit is broken.
A metal box should now be provided to serve as a containing case and the flasher is complete. This box should be of such design and construction that it will comply with the requirements of the electrical inspection department having jurisdiction over the locality where the flasher is to be used.
The flasher should be connected in series with the lamp, the wires being fastened under the nuts on the bolts P and R, Fig. 4, and the screw N adjusted so that it lacks a small fraction of an inch of making contact with the brass plate when there is no current in the winding. When the switch is turned on there will be a current through the lamp and winding in series. The brass strip will be heated more than the iron and it will expand more, thus forcing the point of the screw N down upon the brass plate, which will result in the winding about the brass strip being shorted and the full voltage will be impressed upon the lamp, and it will burn at normal candlepower. When the coil is shorted there will of course be no current in its winding and the brass strip will cool down, the screw N will finally be drawn away from contact with the brass plate, and the winding again connected in series with the lamp. The lamp will apparently go out when the winding is in series with it, as the total resistance of the lamp and winding combined will not permit sufficient current to pass through the lamp to make its filament glow. The time the lamp is on and off may be varied to a certain extent by adjusting the screw N.
Ill: The Assembled Parts Showing the Complete Flasher and Electric Connections with Adjusting Screw