Page:Popular Science Monthly Volume 45.djvu/635

Rh which will be passed into an alternating-current motor driving a direct-current, low-voltage generator furnishing at last the desired electrolyzing current. It has seemed best to submit to this complication of apparatus in order to gain the advantage of entire uniformity and interchangeability of power units in the generating plant. Of course, if the power company were to put in a direct-current dynamo for the benefit of the Reduction Company, all that would be necessary would be to send the current over a wire straight to its work; and it seems remarkable, in view of the thousands of horse power required, that the extra expense of a motor and dynamo to transform this quantity appears preferable. The electrical power unit which has been decided on after the most exhaustive, and presumably competent, expert examination of the requirements of the situation, will be of a capacity for continuous work of five thousand electrical horse power (or three thousand seven hundred kilowatts), and will be directly connected with a pair of turbines of similar power. All the generators will be mechanically identical in construction and have parts interchangeable with each other. The advantage of this, besides the obvious one of having a single set of spare parts suffice against the breakdown of any machine in the station, is that, from a point of view of the electrical aspect of the case, of the machines being able all to be put in parallel, as it is called. The expression may not be a familiar one to some of our readers, and the following hydraulic analogy may be of service in leading to an understanding of what is meant by it. Let us assume that we have several pumping engines of equal power, and that we are using them all to pump water from one reservoir into another at a higher level. Obviously the total amount of water pumped will be what a single machine handles multiplied by the number of them. Had, say, one of the pumps been weaker than the others—had it, that is, not been strong enough to force water up to the height that the others did—the result would be that, instead of doing any work when put, as we may say, in parallel with the others, it would have been unable to withstand the head, and water would have forced itself back through it into the lower reservoir. The same way with dynamos, or generators as they are usually called when referring to the machinery in a power as distinct from a lighting station. The advantage of working in parallel is, that if we have, say, six machines all "pumping" current into the same mains and one breaks down, we may take it out of circuit, and, by temporarily overloading the other five, which can always be done for a short time with good machines, keep on supplying full current to consumers. Should the power company have decided to put in a special machine for aluminum, and other special ones for other local work, and still more for distant work.