although the results vary between rather large limits, certain conclusions may be drawn as to the allowances and the assumptions which may be made in calculating upon acceleration of freight trains.

a. Allowance must be made for an increase in train resistance above the normal. Apparently a resistance of 15 lb. (6.8 kg.) per ton is fair for such conditions as are represented by the tests under discussion.

b. The allowable rate of acceleration may be as low as mi. per hr. per sec. or 10 lb. (4.5 kg.) per ton. This, in combination with 15 lb. (6.8 kg.) per ton for train resistance, is none too low, as the train resistance falls off as soon as the train is under motion, giving a fair rate of acceleration over the range of the rheostat steps.

c. The available tractive effort for acceleration is not more than 80 per cent of the maximum tractive effort at the slipping point of the drivers. This means that if the slipping point is assumed at 30 per cent tractive coefficient, not more than 24 per cent tractive coefficient is available for acceleration.

CONCLUSION

In conclusion the writer begs to repeat what was said earlier, that the results of this paper are not intended to replace any standard formulas on railroad train operation, many of which have been carefully worked out and are supported by abundant experimental data. The paper is presented to show the results of actual tests in actual service and the amount of variation of those results from standard or average values.

The writer also wishes to acknowledge the courtesy of the officials of the Fort Dodge, Des Moines & Southern Railway Co., for extending their facilities to him, and to express his obligation to the students of the senior class in electrical engineering of the Iowa State College who assisted in carrying out the tests and working up the results embodied in this paper.

the American Institute of Electrical Engineers, New York, May 21, 1912.

Copyright, 1912. By A.I.E.E.

THE DEBT WE OWE TO HENRY AS A SCIENTIST

BY MICHAEL I. PUPIN

The name of Joseph Henry is connected with the most brilliant epoch in the history of the science of electricity. To appreciate it fully, let us briefly describe the state of this science prior to the beginning of this glorious epoch-I refer now to the discoveries of the eighteenth century. Stephen Gray's discovery of electrical conduction in 1729 was the broadest foundation for subsequent work. Franklin's discoveries and philosophical speculations in the realm of electrical phenomena gave a tremendous impulse to further research. Galvani's fortunate discovery of the existence of electric force in the contact region of heterogenous bodies closes this period.

Substantial progress was made during the eighteenth century, but the progress was, comparatively speaking, slow and laborious. The intellectual atmosphere of the eighteenth century was rather heavy and quiescent, as if foreboding the approach of a mighty storm.

The storm arrived; the intellectual forces which, like a mountain torrent, broke loose during the French Revolution, and threatened to unhinge every human structure, receded rapidly to their natural channels. The torrent seemed to have washed away every impediment to rapid intellectual progress, and a vigorous advance was started in every direction of human thought. The triumphant forward march of the science of electricity begins now. Volta (1799) discovers the voltaic cell and teaches mankind how to generate electricity in motion. Oersted (1819) discovers the magnetic force exerted by electricity in motion, and Ampere a year later (1820) formulates the fundamental law connecting this magnetic force with the electrical

motion producing it. We have here three giants, Volta, Oersted and Ampere, accomplishing more in twenty years than had been accomplished before in the science of electricity in 2500 years. This kind of work and accomplishment reminds one of the forge of Cyclops as described in Homer's Odyssey. Every stroke shakes the earth to its very foundation. This was, indeed, a stupendous advance, yet it was only the beginning of the great period of electrical discovery and revelation, the period inaugurated by Joseph Henry and Michael Faraday.

Helmholtz and Thomson pointed out many years ago that Oersted's discovery is much broader than Oersted or even Ampere ever suspected, and that it embraces phenomena which these men never dreamt of, the phenomena of electromagnetic induction; or, to use a more descriptive expression, the phenomena of electric forces generated by the motion of magnetism. But to infer, from the existence of magnetic forces produced by moving electricity, the existence of electric forces produced by moving magnetism, it is necessary (as Helmholtz and Thomson point out), to have a clear vision of the principle of conservation of energy. This vision did not appear until nearly thirty years. after Oersted and Ampere had finished their work in electromagnetism. But even if this great principle had arrived prior to the days of Oersted and Ampere, I doubt very much if the astuteness of any human brain would have ever gone so far as to infer, by pure logic, electromagnetic induction from electromagnetism. A logical deduction of this kind would stand today without a parallel in the history of human thought.

The fact that we can, today, in the light of the principle of conservation of energy, see a direct logical connection between electromagnetism and electromagnetic induction, is the best proof that the discoveries of Henry and Faraday furnish one of the most brilliant illustrations of the great power of the principle of conservation of energy.

But Henry's experimental work and Faraday's experimental work had to be done, and their great discoveries had to be made in the very manner in which they made them, in order to reveal before our wondering eyes the beautiful phenomena of electromagnetic induction. Nature guards her secrets too well to reveal a whole world of most startling phenomena to a man who makes no other effort than academic deduction by logic and pure reasoning. Our knowledge of physical phenomena advances by consecutive experimental steps; there

was no direct line from Oersted and Ampere to Henry and Faraday. We had to wait for Arago, who showed that electricity in motion magnetizes a steel needle, and we had to wait for Sturgeon, who showed that an electrical current circulating in a coil of wire wrapped around a horseshoe-shaped piece of steel made a magnet. This was the birth of the electromagnet in 1823. Henry was then a youth, 24 years of age, doing tutoring work in a private family in Albany, and in his leisure hours studying mathematics and reading such works as Lagrange's classical treatise on "Analytical Mechanics." He had never had, up to this time, an instrument for research in his hands, but in less than five years he became the foremost authority, and practically the only authority, on electromagnets. At that time (this was prior to the discovery of electromagnetic induction) the science and the art of the electromagnet was undoubtedly the biggest problem in physics, and the very fact that Henry chose this subject for his study proves that he was cast for a great physicist. Willard Gibbs, our great mathematical physicist, said once that the most essential difference between a great and a commonplace scientist can be seen in the quality of problems which they select. A great physicist knows a great physical problem when he sees it. The electromagnet was a great problem, because it led to the discovery of electromagnetic induction; this was the key, and the only key, which opened the door of the secret chamber within which nature guarded the secrets of electromagnetic induction. Henry found the key and he opened the door which revealed to our wondering eyes the phenomena of electromagnetic induction. At about the same time, and using the same key which Henry had invented, the electromagnet, Faraday, independently of Henry, opened the same door. There can be no doubt as to Henry's claim. Sturgeon said: "Henry has been enabled to produce a magnetic force which totally eclipses every other in the annals of magnetism, and no parallel is to be found. since the miraculous suspension of the celebrated oriental impostor in his iron coffin."

Henry worked day and night making electromagnets that could lift thousands of pounds, and these magnets are still in existence at Princeton and at Yale. If the master-mind which constructed these magnets had not discovered electromagnetic induction, and at the very time when the discovery was made, it would have been a miracle far more wonderful than the discovery itself.