"STEP UP" AND "STEP DOWN" TRANSFORMERS
The dynamos in operation at Niagara do not differ in principle from those in the street-car power-house, except in the fact that they are not supplied with commutators. We have seen that these dynamos are of enormous size. Those already in operation generate five thousand horse-power; others in process of construction will develop ten thousand. The generator which produces this enormous current is about eleven feet in diameter, and it makes two hundred and fifty revolutions per minute. The armatures are so wound that the result is an alternating current of electricity of twenty-two hundred volts. This current represents, it has been said, raw material which is to be variously transformed as it is supplied to different uses. To factories near at hand, indeed, the current of twenty-two hundred volts is supplied unchanged; but for more distant consumption it is raised to ten thousand volts; and that portion which is sent away to the factories of Buffalo and other equally distant places is raised to twenty-two thousand volts.
ELECTRICAL TRANSFORMERS.
The upper figure shows Ferranti's experimental transformer built in 1888. It has a closed iron circuit, built up of thin strips filling the interior of the coil and having their ends bent over and overlapping outside. The lower figure shows a simple transformer known as Sturgeon's induction coil. The middle figure gives a view of the series of converters in the power house of the Manhattan Elevated Railway.
The transformation from a relatively low voltage to the high one is effected by means of what is called a step-up transformer. This is an apparatus which brings into play a principle of electric induction not very different from that which was responsible for the generation of the current of electricity in the dynamo. The principle is that evidenced in the familiar laboratory apparatus known as the Ruhmkorff coil. The transformer consists essentially of a primary coil of relatively large wire, surrounded by, but insulated from, a secondary coil of relatively fine wire. When the interrupted current is sent through the primary coil of such an apparatus, an induced counter-current is generated in the secondary coil. Of course there is no gain in the actual quantity of electricity, but the voltage of the current generated in the finer wire is greatly increased. For example, as we have seen, the current that came from the dynamo at twenty-two hundred volts is raised to ten thousand or twenty-two thousand volts. These proportions may be varied indefinitely by varying the relative sizes and lengths of the primary and secondary coils.
How shall we picture to ourselves the actual change in the current represented by this difference in voltage? We might prove, readily enough, that the difference is a real one, since a wire carrying a current of low voltage may be handled with impunity, while a similar wire carrying a current of high voltage may not safely be touched. But when we attempt to visualize the difference in the two currents we are all at sea. We may suppose, of course, that electrons spread out over a long stretch of the secondary coil must be more widely scattered. One can conceive that the electrons, thus relatively unimpeded, may acquire a momentum, and hence a penetrative power, which they retain after they are crowded together in a straight conductor. But this suggestion at best merely hazards a guess.
Arrived at the other end of its journey, the current which travels under this high voltage is retransformed into a low-voltage current by means of an apparatus which simply reverses the conditions of the step-up transformer, and which, therefore, is called a step-down transformer. The electricity which came to Buffalo as a twenty-two-thousand-volt current is thus reduced by any desired amount before it is applied to the practical purposes for which it is designed. It may, for example, be "stepped-down" to two thousand volts to supply the main wires of an electric-lighting plant; and then again "stepped-down" to two hundred volts to supply the electric lamps of an individual house.
Who that reads by the light of one of these electric lamps, let us say in Buffalo, and realizes that he is reading by the transformed energy of Niagara River, dare affirm that in our day there is nothing new under the sun?