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THE POPULAR SCIENCE MONTHLY. 



It is only by the alternating- system of distribution that we 

 can realize this essential condition of economy. We have here 

 no such limit to the electrical pressure in the generating appa- 

 ratus as in the direct-current system, and through the medium of 

 the converter it becomes possible to vary the two elements of elec- 

 trical energy — current volume and pressure — to suit the most wide- 

 ly differing applications. It is this latter feature of the system 

 which gives it its great range and flexibility, and its consequent 

 economic value. It enables us, for instance, to generate a current 

 of a certain voltage at the machine, then to raise this to ten, twenty, 

 or fifty times the original pressure for transmission through the 

 line, and then at the far end to step down to as low a pressure as 

 we may want — a pressure suitable for entering dwellings, offices, 

 and shops, and safe in the hands of the consumer. These suc- 

 cessive transformations and retransformations, it should be noted, 

 are effected in the simplest kind of a way. They involve no ma- 

 chinery with moving parts, but simply coils of wire placed in 

 such relation to each other that the currents passing in one in- 

 duce similar currents in the other. The practical value of this 

 system arose with the discovery that the induction coil, like the 

 dynamo, is reversible. This coil had long been used to transform 

 a current of considerable volume and low pressure into one of 

 very great pressure and small volume. The construction which 

 enabled this to be done consisted in making the primary coil with 

 a few turns of stout wire ; and the secondary — that on which the 

 induced current was produced — of a great many turns of fine 

 wire. It was presently discovered, however, that this mode of 

 operation might be reversed, and that, by passing a high-tension 

 current of small volume through many turns of wire, a current 

 of large volume and low pressure could be induced in a secondary 

 circuit of few turns, and that the pressure and volume of the in- 

 duced current in relation to that of the primary one depended 

 only on the relative number of wire turns in the two circuits. 

 If, for instance, the primary and secondary coils contained the 

 same number of turns, the pressure and volume of the induced 

 current would be precisely the same as the primary one. If, on 

 the other hand, the induced circuit contained ten times the num- 

 ber of coils of the primary, the current in it would have a tenth 

 of the volume and ten times the pressure of the primary one, 

 while if the relation of the two circuits were reversed the induced 

 current would have its volume increased to ten times and its 

 pressure reduced to one tenth of that flowing in the primary. 



In the field of lighting this method of electric distribution has 

 taken a leading place, and it is no longer questioned that it is des- 

 tined to displace entirely all methods of direct-current supply. 

 It has heretofore found but little application to power transmis- 



