the magnet is represented as heavily shaded. The _magnetic circuit_ is said to be closed from one pole of this magnet to the other through the strips of iron which pass across the ends and down the sides of the coil. The arrows show the path of the magnetic circuit. The dotted portion shows where the copper wire may be supposed to have been cut across. Inasmuch as the electric current is induced in the secondary circuit by continually varying the strength of the magnetic field as much as possible, the alternating current is the most desirable to use in the primary. If the direct current were used an interrupter would be necessary, which would of course produce too much sparking when any but low tension currents are used in the primary circuit. The most interesting and curious fact about the transformer is that the voltages of the primary and secondary currents are in exact proportion to the number of turns in the wire of the two circuits. [Illustration: Fig. 133] [Illustration: Fig. 134] [Illustration: Fig. 135] In our transformer the number of turns in the coil between the binding posts _a_ and _b_ is 400 and the number of turns between _c_ and _d_ is 1200. If now we connect a 112-volt alternating current with the binding posts _a_ and _b_, a volt meter connected across between _c_ and _d_ will show 336 volts, and if _b_ and _c_ be connected by a short wire, bringing in 1600 turns into the secondary circuit, a volt meter connected across between _a_ and _d_ will show a voltage of 448. Or if, leaving _b_ and _c_ still connected by a short wire, we connect the 112-volt alternating current to _a_ and _d_ a volt meter connected across between _a_ and _b_ will show 28 volts, or if connected between _c_ and _d_ it will show 84 volts, and if finally the 112-volt current is connected to _c_ and _d_ the pressure between _a_ and _b_ will be 37-1/3. [Illustration: Fig. 136] The story, then, of the central station which we have chosen as a type is briefly this: Falling water makes dynamos revolve, generating a 4000-volt alternating current. This current is sent through the primary windings of transformers. The secondary windings of these transformers have twenty-five times as many turns as the primary coils. This steps up the voltage from 4000 to 100,000, making it necessary to send only one twenty-fifth as many amperes over the lines as would be required at 4000 volts, and reduces the loss in transmission to nearly one twenty-fifth. At the other end of the line the current traverses the secondary windings of transformers, and the consumer receives his current from primary coils which may step the e. m. f. down to any required volts of pressure, generally 110. Now I shall be glad to have you consider whether this suggests any practicable problems for us here in Millville. The sun is nearly setting and I suppose the family is expecting me home. [Illustration: Fig. 137] XIII ELECTRICITY FROM AN OLD MILL