Inventors at Work, with Chapters on Discovery

by George Iles

Analogy as a Guide.

A conviction that has over and over again served the discoverer assures him that like causes underlie effects which seem diverse. When Thomas Young observed the recurrent bands of darkness due to interferences of light, he at once detected a parallel to the beats by which interferences of sound produce silence. He was therefore persuaded that light moves in waves as does sound, that it is not, as Newton supposed, a material emission. A chapter might be filled with examples of the same kind: let one suffice.

If an ordinary clothes-line, say twenty feet long, receives a wave-impulse from the hand at one end, the motion will proceed to the other end as a series of waves. If a rope twice as heavy is used, a larger part of the original impulse will be received at the remote end than in the first experiment. Of course, there comes a limit to the thickness of the rope which may be thus employed; we must not choose a ship’s cable for instance, but the rope most effective in results is much heavier than one would suppose before trial. Lord Rayleigh, in his treatise on the theory of sound, has shown that according to Lagrange it is unnecessary to thicken a cord when we wish to add to its weight; as an alternative we may fasten weights upon it at due intervals, the whole having less mass than if we used a heavy rope of equal effectiveness. Just what intervals are best will depend upon the thickness and rigidity of the cord, upon its length, the amount and kind of wave committed to it, as shown by Professor Michael I. Pupin of Columbia University, New York, who extended the mathematical problem dealth with by Lagrange and Lord Rayleigh. In the singular efficiency of transmission thus studied he saw a principle which, by analogy, he believed to hold true in the electrical field as in mechanics. This principle he has illustrated in his paper published in the Proceedings of the American Institute of Electrical Engineers, 1900, page 215. In A of the accompanying [figure], derived from that paper, is a tuning fork, C, with its handle rigidly fixed. To one of its prongs is attached a flexible inextensible cord, bd. Let the fork vibrate steadily by any suitable means. The motion of the cord will be a wave motion, as in B. The attenuation of the wave as it dies down is represented in C. Experiments show that, other things being equal, increased density of the string will diminish attenuation, because a larger mass requires a smaller velocity in order to store up a given quantity of kinetic energy, and smaller velocity brings with it a smaller frictional loss. Moreover, as the string is increased in density, its wave-length is shortened.

Prof. Pupin’s diagram explaining his system of long distance telephony.

Suppose now that we attach a weight, say a ball of beeswax, at the middle point of the string, so as to increase the vibrating mass. This weight will become a source of reflections and less wave energy will reach the farther end of the string than before. Subdivide the beeswax into three equal parts and place them at three equi-distant points along the cord. The efficiency of transmission will be better now than when all the wax was concentrated at a single point. By subdividing still further the efficiency will be yet more improved; but a point is soon reached when further subdivisions produce very slight improvement. This point is reached when the loaded cord vibrates nearly like a uniform cord of the same mass, tension, and frictional resistance; such a cord, bearing 12 small weights of beeswax, is represented as D when at rest, as E when in motion. . . . It is impossible so to load a cord as to make it suitable for waves of all lengths; but if the distribution of the loads satisfies the requirements of a given wave-length, it will also satisfy them for all longer wave-lengths.

A cord of this kind has mechanical analogy with an electrical wave conductor. In a wire transmitting electricity inductance coils may be so placed as to have just the effect of the bits of wax attached to the cord in our illustration; in both cases the waves are transmitted more fully and with less blurring than in an unloaded line. The mathematical law of both cases is the same. It was in ascertaining that law so as to know where to place his inductance coils that Professor Pupin arrived at success. Preceding inventors, missing this law, came only to failure. He constructed an artificial cable of 250 sections, each consisting of a sheet of paraffined paper on both sides of which was a strip of tin-foil, the whole fairly representing a cable 250 miles in length. At each of the 250 joints in the course of this artificial circuit he inserted a twin inductance coil wound on one spool 125 millimetres broad and high, and separated by cardboard ¹⁄₆₄ inch thick. Each coil had 580 turns of No. 20 Brown & Sharpe wire. Just as with the weighted rope this circuit transmitted its current much more efficiently than if the inductance coils had been absent.

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.