. The principles underlying radio communication . hout the circuitat every moment. The current in this case, as well as that ofa steady current, may be regarded as like the flow of an in-compressible fluid. The emf. is, however, to be regarded hereas a variable electric pressure, which acts first in one directionand then in the other. ALTERNATING CURRENT. 117 The values of current in the preceding table are plotted inFig. 71 as ordinates (vertically), and the corresponding lengthsof time elapsed since the start, as abscissas (horizontally), anda smooth curve drawn through the points enables on
. The principles underlying radio communication . hout the circuitat every moment. The current in this case, as well as that ofa steady current, may be regarded as like the flow of an in-compressible fluid. The emf. is, however, to be regarded hereas a variable electric pressure, which acts first in one directionand then in the other. ALTERNATING CURRENT. 117 The values of current in the preceding table are plotted inFig. 71 as ordinates (vertically), and the corresponding lengthsof time elapsed since the start, as abscissas (horizontally), anda smooth curve drawn through the points enables one to de-termine what is the value of the current for any moment lyingbetween any two of those which are included in the table. Itis to be noted that the changes of current repeat in the table the current is the same at 1 sec. and 25 the start; at 7 sec. and 31 sec, etc. The interval of 24seconds in this example is the period of this alternatingcurrent. The current passes through a complete cycle ofchanges in one A current like that just treated is the same as that whichwould be produced in a circuit attached to a coil revolving veryslowly in a uniform magnetic field. (See Chap. 2, Sec. 74.)The motion has been assumed slow in order that the changescan be followed with ordinary direct-current instruments. Inorder to represent the current developed by an ordinary low-frequency alternating-current generator, we must, however,imagine the coil to revolve more than a thousand times morerapidly. Thus the usual a. c. lighting circuits carry currentswhose period is only about -fa second. The current passesthrough complete cycles each second, that is, its frequency is 60 cycles per second. Ordinary alternating-current gen-erators can not use magnetic fields which are entirely uniform,so that the current obtained never passes through its changesin exactly the same way as the ideal sine current pictured in 118 ELEMENTARY ELECTRICITY. Fig. 71. The differenc
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