In 1893, Oliver Heaviside proposed that the inductance of telephone lines be increased above the amount natural for the inter-axial spacing, with a view to counteracting the hurtful effects of the capacity.  His meaning was that the increased inductance—­a harmful quality in a circuit not having also a harmfully great capacity—­would act oppositely to the capacity, and if properly chosen and applied, should decrease or eliminate distortion by making the line’s effect on fundamentals and harmonics more nearly uniform, and as well should reduce the attenuation by neutralizing the action of the capacity in dissipating energy.

There are two ways in which inductance might be introduced into a telephone line.  As the capacity whose effects are to be neutralized is distributed uniformly throughout the line, the counteracting inductance must also be distributed throughout the line.  Mere increase of distance between two wires of the line very happily acts both to increase the inductance and to lower the capacity; unhappily for practical results, the increase of separation to bring the qualities into useful neutralizing relation is beyond practical limits.  The wires would need to be so far above the earth and so far apart as to make the arrangement commercially impossible.

Practical results have been secured in increasing the distributed inductance by wrapping fine iron wire over each conductor of the line.  Such a treatment increases the inductance and improves transmission.

The most marked success has come as a result of the studies of Professor Michael Idvorsky Pupin.  He inserts inductances in series with the wires of the line, so adapting them to the constants of the circuit that attenuation and distortion are diminished in a gratifying degree.  This method of counteracting the effects of a distributed capacity by the insertion of localized inductance requires not only that the requisite total amount of inductance be known, but that the proper subdivision and spacing of the local portions of that inductance be known.  Professor Pupin’s method is described in a paper entitled “Wave Transmission Over Non-uniform Cables and Long-Distance Air Lines,” read by him at a meeting of the American Institute of Electrical Engineers in Philadelphia, May 19, 1900.

NOTE.  United States Letters Patent were issued to Professor Pupin on June 19, 1900, upon his practical method of reducing attenuation of electrical waves.  A paper upon “Propagation of Long Electric Waves” was read by Professor Pupin before the American Institute of Electrical Engineers on March 22, 1899, and appears in Vol. 15 of the Transactions of that society.  The student will find these documents useful in his studies on the subject.  He is referred also to “Electrical Papers” and “Electromagnetic Theory” of Oliver Heaviside.