Construction of a 2-K.W. Transformer.
Core.—Strips 2 1/2 inches wide are cut from soft sheet iron. One half of them should be 11 1/2 inches long and the other half 6 1/2 inches. Enough are cut to make two piles of each size 2 1/2 inches high. Both sides (the longest strips) of the core are built up with the ends overlapping as in Fig. 41.
The ends (the short pieces) are then slipped between the overlapping ends of the long strips and the whole core squared up. The completed core should have a cross section of 2 1/2 inches square and form a hollow rectangle 8 x 14 inches. The strips of iron must be dipped in P. & B. insulating varnish and dried before they are assembled.
Primary.—Four fiber heads 6 inches square, 1/2 inch thick and having a hole 2 1/2 inches square are made. One of these is placed on the core legs as shown by Fig. 48. Two or three layers of well varnished linen are wound over the core preparatory to winding the primary. Room must be left for the other head to be placed on the opposite end after the windings are all in place. The primary consists of 100 double turns of No. 12 B. S. gauge double cotton covered magnet wire. Fifty of the double turns are wound on each leg. The four terminals of the primary are led out through the fiber heads. Care should be taken to distinguish them from each other so that they may be identified when the transformer is completed. The primary is then wound with a strip of micanite or empire cloth 8 inches wide until it measures 4 inches square over all.
Secondary.—The secondary coils, which are eight in number, are wound on a form 4 inches square and 1 1/2 inches between the flanges. The construction of the form is similar to that shown in Fig. 46 but is larger. The slots are also necessary here so that the completed section may be tied up.
About ten pounds of No. 30 B. S. gauge single silk covered wire are required to wind the secondary. The sections are wound in smooth even layers until they are six inches in outside diameter. They are then tied up and removed from the winder. The sections are separated by sheets of fiber 6 1/2 inches square on the outside, 1/2 inch thick, having a hole 4 inches square cut in the center. The sections are all connected in series and the terminals soldered to strips of copper.
After the secondary coils are all in place and connected, the fiber head is slipped on the end of the leg. Then the short lengths of the core, which had been pulled out again after squaring the core up, are slipped into place. The core is squared up again and fastened together by boring a 3/8-inch hole completely through the core at each corner. Two strips of fiber 12 x 2 1/2 x 1/2 inches are bored with corresponding holes in their ends. These strips are placed at the end of the transformer, on top of the core, and 1/4-inch bolts passed through the holes in the fiber and the core. The bolts are wrapped with micanite cloth where they pass through the core, and an insulating washer is placed under the nuts, so that the iron core laminations are not electrically connected. The nuts are tightened until the core is held firmly together.
The fiber strips also serve as insulated supports for the binding posts. The copper terminals of the secondary lead to two binding posts mounted on two fiber or hard rubber pillars 1 inch diameter and 4 inches high. The rods are arranged as explained in the section under the heading of a 1/4-K.W. transformer.
The transformer is designed for use on a 60-cycle 110-volt current. It may be used on 220 volts if the two primary coils are connected in series. When the primaries are in series, and the transformer is used on the 110-volt current, it will deliver a voltage of about 12,000 at the secondary. With either primary alone the voltage will be about 12,000, and with both in parallel about 25,000. It will then deliver a very heavy current at the secondary and draw from the line about 20 amperes in the primary. If used with a proper tuning helix, condenser and aerial, the transformer is capable of sending about 300 miles under favorable conditions.
If the transformer is to be used for long periods at a time, it is best to place it in a tight wooden box 18 inches square and 12 inches deep. The box is then filled with boiled linseed oil or amber petroleum.