The distance to a great extent depends on the cost of coal required for generation at the distributing point and on the amount of energy demanded at the receiving point. Of course the farther the distance the higher must be the voltage pressure.
Electrical engineers say that under proper conditions electric energy may be transmitted in large quantity to a distance of 500 miles and more at a pressure of about 170,000 volts. If such right conditions be established then New York, Chicago and several other of our large cities can get their power from Niagara.
In our cities and towns where the current has only to go a short distance from the power house, the conductors are generally placed in cables underground and the maximum electro-motive force scarcely ever exceeds 11,000 volts. This pressure is generated by a steam-driven alternating-current generator and is transmitted over the conductors to sub-stations, where by means of step-down transformers, the pressure is dropped to, say, 600 volts alternating current which by rotary converters is turned into direct current for the street mains, for feeders of railways and for charging storage batteries which in turn give out direct current at times of heavy demand.
That electric transmission of energy to long distances may be successfully carried out transformers are necessary for raising the pressure on the transmission line and for reducing it at the points of distribution. The transformer consists of a magnetic circuit of laminated iron or mild steel interlinked with two electric circuits, one, the primary, receiving electrical energy and the other the secondary, delivering it to the consumer. The effect of the iron is to make as many as possible of the lines of force set up by the primary current, cut the secondary winding and there set up an electromotive force of the same frequency but different voltage.
The transformer has made long distance the actual achievement that it is. It is this apparatus that brought the mountain to Mohammed. Without it high pressure would be impossible and it is on high pressure that success of long distance transmission depends.
To convey electricity to distant centres at a low pressure would require thousands of dollars in copper cables alone as conductors. To illustrate the service of the transformer in electricity it is only necessary to consider water power at a low pressure. In such a case the water can only be transmitted at slow speed and through great openings, like dams or large canals, and withal the force is weak and of little practical efficiency, whereas under high pressure a small quantity can be forced through a small pipe and create an energy beyond comparison to that developed when under low pressure.
The transformer raises the voltage and sends the electrical current under high pressure over a small wire and so great is this pressure that thousands of horse-power can be sent to great distances over small wires with very little loss.
Water power is now changed to electrical power and transmitted over slender copper wires to the great manufacturing centres of our country to turn the wheels of industry and give employment to thousands.
Nearly one hundred cities in the United States alone are today using electricity supplied by transmitted water-power. Ten years ago Niagara Falls were regarded only as a great natural curiosity of interest only to the sightseer, today those Falls distribute over 100,000 horse-power to Buffalo, Syracuse, Rochester, Toronto and several smaller cities and towns. Wild Niagara has at last indeed been harnessed to the servitude of man. Spier Falls north of Saratoga, practically unheard of before, is now supplying electricity to the industrial communities of Schenectady, Troy, Amsterdam, Albany and half a dozen or so smaller towns.
Rivers and dams, lakes and falls in all parts of the country are being utilized to supply energy, though at the present time only about one-fortieth of the horse-power available through this agent is being made productive. The water conditions of the United States are so favorable that 200,000,000 horse-power could be easily developed, but as it is we have barely enough harnessed to supply 5 million horse-power.