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Page:Popular Science Monthly Volume 18.djvu/508

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small motors attached to the various pieces of apparatus to be driven. But generally, in cities where power could be distributed, people would prefer to have it furnished without thought or care on their part about its production.

For such a power, one that can at any time be increased to meet the utmost demands, we must, therefore, look to some other agency than water. Compressed air is, without question, an available one, and the motors in which it could be used are comparatively simple, but, as it could only be employed for this one purpose (unless, indeed, sanitary advantages were realized), the present or prospective demand would hardly warrant its adoption. The agent that appears to be the most suitable, and that gives promise of utility in other directions as well, is electricity. Distributed from a central source of supply, all the advantages of a safe and convenient power would be obtained, without any of the disadvantages attendant upon the use of other forms of energy. Of the feasibility of economically distributing the electric current there is a growing confidence among electricians, and the advantages of so transmitting power have been frequently urged of late, not only for moving light machinery, but for doing all the work now done in our factories by the steam-engine.

Distribution accomplished, the machines by which the current is utilized are very simple, and need not be expensive. As remarked by Dr. Paget Higgs, they are so much cast-iron and insulated copper wire, and their construction requires none of the skilled work necessary in the various forms of heat-engine. The construction is practically the same in the essential parts, whether they be used as current generators or as motors. Briefly, such a machine consists of one or more electro-magnets placed so as to revolve before and very close to the poles of another electro-or permanent magnet, the former system of magnets being: termed the armature, and the latter the field.

When permanent magnets are used for the field, the machines are known as magneto-electric, and, when these are replaced by electromagnets, as dynamo-electric. The operation of both kinds depends, as is well known, upon the inductive action between the armature and the field magnets, a current being induced in each of the coils of the former as they approach, and an equal and opposite one being set up as they recede from the poles of the latter. In dynamo-machines the magnetization of the field is due to the currents generated by the machine itself. The soft-iron cores, after they have once been magnetized, always retain some residual magnetism which serves to induce a feeble current in the armature. A portion of this is sent through their coils, increasing their magnetization, which in turn augments the strength of the induced currents, and thus, by this successive action and reaction between the field and the armature, a very powerful magnetization of both is shortly produced. The currents are usually collected-in such a manner that they both have the same direction in the circuit, by a sim-