248 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



be a continuous current of constant value. Energy can be steadily 

 extracted from such a system only by introducing some apparatus 

 connected with the wire which is the guiding core for this energy. 



Let us now consider the case of current of a different character, a 

 fluctuating — or better, an alternating current. Let us substitute for 

 the battery an alternating current generator, and assume a single 

 wire with an earth or wire return, as in figures 3 and 5. Here the 

 wire merely becomes positive and negative alternately, for the circuit 

 is incomplete or unconnected as a circuit, and the stress lines from 

 wire to earth or to other Avires reverse periodically their direction 

 plus to minus and minus to plus. This is true, of course, whether 

 the earth be replaced by a second wire or whether three or more wires 

 be involved, as in a three-phase alternating current circuit. By 

 connecting any two of the wires through an energy-receiving ap- 

 paratus R (fig. 11), the same ac- 



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tion that takes place with the 

 continuous current may be repro- 

 duced except that the energy now 

 comes in waves and is not a con- 

 tinuous flow. In ordinary cases 

 there are 60 complete waves or 

 complete changes from plus to 

 minus and back to plus in each 

 second, and the system is then 

 called one of 60-cycle frequency. 

 A further important difference is 

 to be noted between the alternat- 

 ing-current condition and the 

 continuous. The action in the 

 ether around and between the 

 wires is now in the form of waves, both magnetic and electrostatic. 

 Between wires there is an increase of electrostatic stress to a maxi- 

 mum, a diminution to zero, a reversal, etc. The magnetic field 

 also rises, falls, reverses, and so on synchronously. The condition 

 is no longer static, the medium around the wires is in a dynamic 

 state and it is now possible to abstract energ}^ steadily from it 

 without actually diverting cun-ent from the line. We can, in fact, 

 by such a system produce in neighboring conductors similar disturb- 

 ances or currents, and along with these disturbances we may deliver 

 energy. 



The alternating-current transformer is then merely a device for 

 l)ringing two or more circuits together as near as possible and en- 

 hancing the magnetic values which would normally exist around 

 such circuits by the addition of an iron atmosphere, the iron core, 

 so that the greatest possible transfer of energy from one (the pri- 



