492 THE POPULAR SCIENCE MONTHLY, 



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 iirged 

 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 neces- 

 sary in the various forms of heat-engine. The construction is pi*acti- 

 cally 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 masrnets 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 electro- 

 magnets, as dynamo-electric. The operation of both kinds depends, as 

 is well known, lapon 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 ma- 

 chine 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, in- 

 creasing their magnetization, which in turn augments the strength of the 

 induced currents, and thus, by this successive action and reaction be- 

 tween 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- 



