It is not thought to be advisable to raise the voltage at the generator higher than 4000. This will not suffice to supply large working currents to a greater distance than about six or eight miles. For a distance of 10 miles 6000 volts are desirable; for 50 miles 30,000 volts; for 100 miles 60,000 volts; for 165 miles 100,000 volts; and for 200 miles 120,000 volts. Notice that in this table the voltage rises at the rate of 600 per mile. Since it is not desirable for the generator itself to produce a higher voltage than 4000, we must depend upon transformers to produce these high voltages. Let us then consider, a little more in detail, the construction of a transformer. I have here some drawings of one which I propose that we make in the machine shop, and use in our central station equipment in the future. We will procure the thinnest and softest sheet iron possible and cut out of it a lot of pieces shaped like the letter H with the dimensions shown in Fig. 131. These are to be piled one upon another, with strips of paper between, until the pile is 1-1/2 inches thick. Then four pieces of board are to be bolted to the sides of these (Fig. 132). The dimensions of each of the four blocks, is to be 7-1/2 inches long by 3 inches wide by 1-1/2 inches thick. Upon the cross bar of the H we will wind 400 turns of No. 12 double cotton-covered copper wire, bringing out the ends for future attachments, and then wind on 1200 turns of No. 10 double cotton-covered copper wire. The wire will fill the space between the blocks as indicated by the diagram in Fig. 133. We will then cut strips of the sheet iron 6 inches long by 1-1/4 inches wide, and bridge across the ends of the H, prying open the leaves of sheet iron and tucking them in between as shown in Fig. 134. We shall then drill a hole at each corner and bolt them in place. Binding posts will be placed at _a_, _b_, _c_, and _d_ (Fig. 134), and the two ends of the No. 12 wire will be brought to _a_ and _b_ and those of the No. 18 wire will be brought to _c_ and _d_. Going through all this detail of construction has probably made you lose sight of the essential features of this transformer. Let us for a moment turn back and see what they are. We have a large coil of wire 3 inches long and 7-1/2 inches in diameter. It is composed of a coarse winding and a fine winding, which we may designate as the primary and secondary coils, if we choose. Of course the only reason for having different sizes of wire is so that we may send larger currents through one than the other. The coil has a laminated iron core, that is, it is composed of layers of sheet iron. These layers are insulated from one another. This is essential, although we cannot explain why now. But perhaps the most essential feature of the transformer is that iron extends clear around from one pole of this electro-magnet to the other. Fig. 135 represents a section through the coil made in the plane of _e f g_ (Fig. 134). The core of