It not infrequently occurs that where one has alternating current at no volts available, a lower voltage is required for some special purpose, as the operating of small lamps of low voltage, induction coils, etc. In theory it is a very easy matter to construct a small step-down transformer to reduce the voltage and increase the quantity, but in practice many difficulties arise. The windings together with the size and mass of iron in the core all have to be taken into consideration to obtain satisfactory results. The writer has recently had occasion to construct a transformer for the purpose of stepping down an alternating current of no volts and 60 cycles, to furnish a current to replace that from a series of four to six dry cells for operating a gong-striking device, the circuit of which is closed at intervals by a clock, and the current carried through an electro-magnet originally wound to a resistance of 4 ohms, this in turn attracting an iron armature with hammer, striking the gong. Because the batteries would run down rapidly, it was deemed of advantage to make use of the lighting current already on the premises, in connection with the transformer referred to. As the primary windings were connected across the 110-volt mains at all times, and the secondary only closed at intervals, it was important that as little current as possible pass through the primary windings and register on the meter, when the secondary was open. A sufficient number of pieces of soft stovepipe iron were cut each 6 1/2 inches long and 1 1/4 inches wide, to make two piles, each 1 1/4 inches high; and a number of pieces 5 inches long and 1 1/4 inches wide were also cut to make two piles, each 1 1/4 inches high. These were well coated with shellac varnish, and then arranged as shown at C in Fig. 225, forming two right angles, the shorter pieces being placed between the longer pieces at one end in each case. The corner of the pile in each case was then squeezed down in a vise, and the iron well wrapped with adhesive insulating tape, putting on several layers, but not covering the outer ends for a space of 1 3/8 inches. After releasing the vise, that portion at the angle-was also covered with tape. The secondary coils were then put on, consisting of 100 feet of No. 16 double cotton magnet wire, on each of the angle pieces, or 200 feet altogether, and well coated with shellac, the windings being in such direction that when the two angle pieces were brought into contact at the exposed ends, they formed a continuous magnetic circuit. The windings were all in one direction. These secondary windings were then covered with two layers of cotton cloth, well coated with shellac varnish, and the primary windings were put on over the secondary, consisting of 650 feet of No. 28 double-covered cotton magnet wire on each side, or 1,130 feet altogether, wound in the same manner as the secondary, and coated with shellac varnish. The exposed corners of the two iron cores were then brought together, and clamped firmly, as shown, with iron clamps B, and then the whole was mounted horizontally on four large porcelain knobs screwed on a suitable wooden base. This served to insulate it, and allow for the passing off of any heat generated. It was held in place with strips of tape passing down around the knobs. The primary coils (the No. 28 wire) were now connected in scries, the ending of the coil on the first side wound, being connected with the beginning of the winding on the last side wound, the ending of the last coil, and the beginning of the first being connected to tin- 110-volt, alternating circuit, being bridged across it, not cut in in series. The secondary windings were connected experimentally in two ways: first, in series, giving 200 feet from the terminal of the first winding to the commencement of the last winding; and afterward in parallel, the beginning of the first and second windings being twisted together, and the endings of the first and last windings, giving a length of 100 feet of wire, and a decrease of resistance due to the mass of copper. Both methods of connecting worked well, the series connecting giving 14 volts and about 5 amperes, and the parallel 7 volts and about 10 amperes. For the particular purpose for which this was constructed, it is giving excellent results, there being very little heating, even when the secondary is closed, and practically none when it is open.

Fig. 225 - Construction of the small transformer.

It was found advisable to change the winding of the electromagnet that strikes the gong, owing to the counter electro-motive force generated in it by the alternating current, and it was rewound with 60 feet of No. 17 double cotton magnet wire. Its cores were about 5/8 inch in diameter, and 2 1/2 inches long. It is well to remember that in operating electro-magnets with the alternating current, a considerably higher voltage will have to be used than in the case of the direct current, owing to the choking effect due to counter electromotive force, and that the cores should be laminated. Where a current of higher voltage is required, particularly if the transformer is only required to be connected with the lighting mains at intervals for operating large induction coils, or charging storage batteries through a rectifier. etc., it may be made in the same general form as the one just described, with the following changes: Make the pieces for the core 7 inches long and 1 1/2. inches wide, and 5 inches long and 1 1/2 inches wide, and of such quantity that they will make two piles in each case, of a height of 2 inches, and coat with shellac as before, and assemble, etc. Wind the secondary with 60 feet on each side, or 120 feet altogether, of No. 14 double cotton magnet wire well coated with shellac. Put on two layers of cloth well coated with shellac, and wind on the primary coils over the secondary, consisting of 150 feet on each side of No. 18 double cotton-covered magnet wire, or 300 feet on both sides, and coat well with shellac. Clamp the two sides together, as in the previous case, they being in this case 2 inches thick, and mount as in the previous case. Connect the primary windings in series, the terminal of the first winding to the beginning of the second winding, and the beginning of the first winding and the terminal of the second to wires leading to the no-volt feed wires, to which they are connected as in the previous case. The secondary windings are connected in series, in the same manner as the primary, and a short tap is run out from the point where the two coils are connected together, the beginning of the first winding and the terminal of the last winding being connected to the circuit where the current is to be used. This transformer connected in this way will give a current of 40 volts and a maximum amperage of 10. By making connection with the beginning of the first winding and the tap between the two windings only, the voltage will be 20; and by connecting the two windings in parallel, the two beginning wires of the two windings being twisted together and the two terminal wires of the two windings being likewise twisted together, the current will be 20 volts, with a considerable increase in amperes as compared with the connection in series. It will be noted that the secondary winding is put on first, the primary being wound over it, the primary being considered as being the current of highest voltage coming from the mains to be stepped down, and the secondary being the winding producing the current of reduced voltage. An increase in the length of the secondary winding increases the voltage of the current produced, and a shortening of the secondary winding reduces the voltage; but if any very radical change is made in the winding, it may necessitate a change in the amount of iron in the core to get the best results. Pieces of fiber or wood may be shaped to fit, and slipped over the core to aid in holding the windings in place if desired, forming spools as it were. It is advantageous to extend the windings, as shown, on both sides of the angle, as thus the whole or nearly the whole of the magnetic flux is made to pass through or thread the coils; but in the case of the transformer last described, the amount of wire being less and being closer to the core, the windings may be on the two opposite sides only, the shorter sections of the core merely completing the magnetic circuit, and not being covered with wire. Both 1 of these transformers are intended for use on a single-phase alternating current of no volts, and of a frequency of 60 cycles. The one first described, when the secondary is open, uses very little current, rather less than that required for a four-candle-power lamp. The normal primary current of the last-described transformer is 2 amperes with proper load.