Page:Popular Science Monthly Volume 73.djvu/312

308 each turbine, under an effective head of about 140 feet, dashes against the turbine blades, the water is disgorged into a subterranean tunnel about 21 feet in height and over a mile in length, through which the discharged water is conducted to the lower Niagara River, just below the abutment of the upper steel arch bridge.

The turbines, by means of the shaft connection, cause the generators on the power-house floor to revolve at the rate of 250 revolutions a minute and to develop a two-phase alternating current of 5,000 horse-power.

Each generator with its connected turbine is entirely distinct from the other generators and turbines, and can be stopped by shutting off the water from the supplying penstock and by applying electrically operated brakes.

Practically, the whole tremendous weight of each generator, shaft and turbine is sustained by the hydrostatic upward pressure of water conveyed in separate pipes from the level of the intake canal to a compartment of the turbine wheel-case where the water presses against the lower surface of a disc secured to the shaft.

To generate an electric current by dynamos a coil of wire must cut the lines of magnetic force emanating from a magnetic field. It is immaterial whether the coils of wire (or armature) revolve around the magnetic field or the magnetic field revolves around the armature. For mechanical reasons, however, all the great generators at Niagara Falls are constructed so that their magnetic fields shall revolve about the coils of wire (the armature).

After the generators have developed the electrical energy the current controlled by appropriate switching devices is transmitted to the power tenants in the immediate vicinity and to the more distant tenants in Buffalo, Lockport and elsewhere. Where power is delivered to tenants within a short radius of the power plants, the current is transmitted at the generator voltage, but where power tenants are situated at such distances from the generators that electric current could be transmitted only at considerable loss at the generator voltage, it becomes necessary for economical transmission to increase or "step up" the voltage of the current by passing the electrical flow through transformers. A transformer by means of primary and secondary coils of wire of different diameters wound around a laminated iron core and with windings in a fixed ratio to each other may increase or diminish the voltage of a current according as an increase or a diminution of voltage may be necessary. The voltage of the current delivered in Buffalo is about 22,000, but as it would be obviously impossible to utilize so great a current pressure the electricity in Buffalo again passes through transformers where the voltage of the current is reduced to that required in the electric light and electric car service and for other power purposes.