This artificial cable, when without coils, through a distance equal to fifty miles of ordinary line worked well, up to seventy-five miles it served fairly well, but proved impracticable at 100 miles, and impossible at distances exceeding 112 miles: all this in exact correspondence with an actual line of the same length. Over a uniform telephone line an increase of distance interferes with the transmission of speech, not only by diminishing the volume of sound, but also from the rapid loss of articulation. At first this manifests itself as an apparent lowering of vocal pitch. In Professor Pupin’s experiments an assistant’s voice at the end of 75 miles of uniform cable sounded like a strong baritone; at 100 miles it became drummy so that it was understood with difficulty, although the speaker had his mouth close to the transmitter, and spoke as loudly as if he were addressing a large audience. At more than 112 miles nothing but the lowest notes of his voice could be heard, the articulation was entirely gone. As soon as the coils were inserted the drumminess ceased, and conversation could be carried on as rapidly as one chose through the whole circuit of 112 miles. Drumminess is due to the obliteration of the overtones, long distance transmission weakening these overtones much more than it does the low fundamental tones. The addition of coils makes the rate of weakening the same for all vibrations, hence the transmitted sound has the same character at the end of the line as at the beginning.
In practice Professor Pupin’s method has proved a remarkable success. In ordinary circuits it reduces materially the quantity of wire necessary. Where a circuit is unusually long it assures clearness of tones or of signals at distances previously out of the question. It makes possible telephony across the Atlantic: a cable for this service would cost only one fourth more than an ordinary telegraphic cable as now laid and used. A decided advantage is reaped by its use in underground cables, liable as they are to a serious blurring of currents at distances comparatively short. The intervals at which inductance coils should be placed depend upon the circumstances of each case. These are discussed by Professor Pupin in the paper here mentioned.
Rules that Work Both Ways.
Analogy in many a path such as that of Professor Pupin has served as a guide to the discoverer and inventor. Equally gainful has been the conviction that many rules work both ways, so that ingenuity has only to execute the converse or the reverse of a familiar task in order to abridge toil, or reach a prize wholly new.
A crow wishes to get at a clam which it has dug out of the sand. To break the stout shell is beyond the strength of its bill, so the knowing bird flies aloft, lets the clam fall on a rocky beach or a stone and forthwith enjoys a meal. It makes no difference whether a hammer falls on the shell, or the shell falls on a hammer: the crow takes the one method within its power. So with the wood-chopper whose axe becomes imbedded in a stick of birch or maple: he lifts wood and axe together as high as he can, then lets the axe fall on its back, when the shock instantly tears the stick apart. Drilling in a lathe is usually executed by the screw of the poppet advancing during the process. In boring long holes, the object to be bored is rotated and moved in a straight line, while the tool advances without revolving. In an emergency William Fairbairn, the famous engineer, had in hand a large task of riveting. He took a punching machine, reversed its action, and had a riveting machine which turned out work twelve times as fast as a skilful workman.
As in the machine shop so in transportation. One of the notions of the pioneer railway engineers in England was that their rails must be flanged, for how else could wheels remain on the track? But somebody with breadth of view-point asked, Why not leave the rail flat, or nearly so, and put the flange on the wheel, an easier thing to do? Accordingly to the wheel the flange went and there it stays, to remind the traveler of the Eastern maxim: “To him who is well shod it is as if the whole world were covered with leather.”
In many tasks we have a like choice of methods. We wish to measure the velocity of a stream; if we immerse a bent glass tube so that its horizontal part is upstream, the height to which the water rises in the upright half of the tube will tell us what we wish to know; if we reverse the tube, a sinking instead of a rising in the upright glass will measure the speed of our current.
Water heightened
in tube.
Water lowered
in tube.