Fig. 15—Wood Filler
The secondary, which is the low-voltage side in this case, as you are using the transformer to reduce or step down the voltage, will have the smaller number of turns, and larger wire should be used in winding it than in the primary, as it will carry a larger current. On account of the secondary being of larger wire, it will be placed on the core first. For this winding you will need a small quantity of No. 26 B. & S. gauge, single cotton-covered wire. Drill a small hole through one of the insulating washers, down close to the cloth covering the core, being careful at the same time to keep the hole as far from the metal part of the core as possible. Pass the end of a short piece of No. 18 or 20 B. & S. gauge, double cotton-covered wire through this opening and solder it to the end of the No. 26 wire. Insulate the joint with a piece of paraffin paper or cloth, and bind the piece of heavy wire to the core of the transformer with a piece of linen thread.
Now wind the No. 26 wire on the core as evenly as possible, to within about 1/8 in. of the end of the spool. Place over the first layer two layers of paraffin paper and wind on a second layer of wire. Three layers should give you the required number of turns in the secondary winding and a resistance of approximately 3-1/2 ohms. The end of the secondary winding should be terminated in the same way as the winding was started. Outside of the completed secondary winding place at least six layers of paraffin paper, or several layers of insulating cloth. The paraffin paper used should be approximately five mills in thickness. You can make your own paraffin paper by taking a good quality of writing paper about two mills thick and dipping it into some hot paraffin, then hanging it up by one edge to drain.
The primary winding is to be made from No. 34 B. & S. gauge, single silk-covered copper wire. The inside end of this winding should be started in the same way as the secondary, but at the end opposite to the one where the secondary terminated. Wind about 240 turns on each layer and place one layer of paraffin paper between each layer of wire. The primary winding should have at least 12 layers, and the outside end should be terminated as the inside end. Outside of the completed windings, place several layers of insulating cloth to serve as an insulation, and at the same time provide a mechanical protection for the windings.
Fig. 16.—Upper Clamping Pieces
The outside part of the magnetic circuit can now be put in place. When the U-shaped pieces are all in place, the magnetic circuit will have the form and dimensions shown in Fig. 11. A clamp should now be made for each end of the transformer, to hold the pieces forming the magnetic circuit together, and at the same time give an easy means of mounting the transformer. Cut from a piece of sheet iron, about 1/16 in. in thickness, two pieces whose dimensions correspond to those given in Fig. 16, and two pieces whose dimensions correspond to those given in Fig. 17. Drill the holes in these pieces as indicated, and bend the larger ones into the form shown in Fig. 18. These pieces can now be clamped across the ends of the transformer with small bolts, as shown in Fig. 19.
Fig. 17—Lower Clamping Pieces and Mounting Supports