ALONG CoNnut ions. lf the E. Al. F. impressed upon a conducting circuit is not constant. the current will vary and there will ho complications owing to the induced currents. In part ienlar, if the E. AI. F. is periodic in the saint' way that the motion of a pendulum is periodic. i.e. if it rises to a maximum. decreases to zero. lwecomes nega tive. reaches a inaximum negative increases to zero, then to its positive maximum, etc.. ac cording to the formula E=E2ens pt„ the E. 11. and the eurrent are the simplest eases of 'alter nating' clectro-motive forces and currents. E" is the maximum E. Al. F.. and the number of times the E. AI. F. makes a complete period in one second is p/..271. where 7r= 3.14159). if the con ductor to which this alternating E. 31. F. is ap plied is not too long, the intensity of the current in all part: of it will be the same at any one time; but_ if the conductor is many miles long— like an ocean Cable, or a telephone or telegraph line over long distanecs—the E. NI. E. and the intensity of the current will vary from point to point in suede a way that there is a wave of E. Al. E. and of current in the conductor. (These
fact: are perfectly analogous to the observed on shaking sidewise periodically fine end of a short rope or of a long sin-It-bed rope.) The rate of transmission of the electrical waves along the conductor depends upon the properties of the csaiductor and the surpmnding medium: and it can be proved that the rate at which the ampli t ink. of the wave—that is. the maximum value of the current at any one point—flecrea,:es owing to absorption of the energy of the waives varies with the wavelength. Long waves, that is those for which p is small, die dour more slowly than short waves, for ttlricIt p is large. It is owing to this fact that long waves travel farther than short ones; and therefore on ocean cables very long electrical waves are used. A discovery made iu Halo by Professor Pupil' of Columbia Univer sity has shown a method by whieh a special kind of conductor—one in which coils of wire arc in troduced at regular distances—will convey all waves (within certain limits) to the snow dis tance, and will convey them much farther than will au ordinary wire or cable.