a freely suspended magnet, thus establishing the relation between magnetism and electricity which many of the ablest philosophers had sought in vain for years. Ampère first heard of what was called the "Copenhagen experiment" on September 20, 1820. On the 18th of the same month he presented to the French Academy a paper in which he announced the fundamental principle of the science of electro-dynamics, together with a number of capital experiments in extension of Oersted's principle. In the incredibly short time of a single week he had gone all over Oersted's work, experimentally and theoretically; he had devised a new and ingenious hypothesis, for the examination of which he had invented novel forms of apparatus, and by means of which he had brought the whole subject within the domain of mathematical treatment. The history of the science of electricity shows nothing more brilliant than the work of that memorable week. To him who was first to show the action and reaction of currents upon each other, and at the same time furnish a rational and most useful hypothesis upon which the now rapidly growing theory of electromagnetism might be constructed, has long been freely accorded the high praise which is implied in calling the unit of current measure an ampere.

The beautifully simple law of Ohm, to which reference has already been made, and which is as omnipresent and omnipotent in electricity as is Newton's law of gravitation in astronomy and mechanics, is administered by and through a triumvirate. Two of the triad, namely, resistance and current, are presented above. The third, which is mathematically the product of these two, is the electro-motive force in the circuit, and its unit of measure is the volt. The appropriateness of this name will be at once recognized when the services of the distinguished Italian philosopher, Volta, the contemporary of Gal

vani, are remembered. In his early youth Volta was considered dull, and he showed little promise of future distinction. His first awakening to intellectual activity manifested itself in a tendency to compose poetry, but from this he turned to experimental science; and when Galvani, in 1786, saw in the twitchings of the legs of a frog the beginning of a series of marvelous discoveries which have made the nineteenth century greater than any that have gone before, Volta was in the prime of life, thoroughly equipped by taste and experience to take up the subject at a point where his countryman seemed likely to leave it, and so enlarge and enrich it as almost to make it entirely his own.

Differing from Galvani as to the cause of what was long called "galvanism," he originated what is known as the "contact theory," and was the first to have clear ideas of what is now termed "electro-motive force." His theory led him to the construction of the voltaic pile or battery, which has been of incalculable value in the development of the science of electricity and its applications. happens that the unit of measure, one volt, is very nearly the electro-motive force of one cell of Volta's battery, being a little less than that of an ordinary sulphate of copper ("bluestone ") cell.

It

These, the ohm, the ampere, and the volt, are the three fundamental units of electrical measurement. They are related to one another through Ohm's law, and, as other units are largely derived from them, it will be useful to il lustrate this relation before proceeding further. For this purpose, perhaps nothing is better than the well-known and oft-repeated comparison of the flow of a current of electricity in a conductor to the flow of a stream of water through a pipe. When water flows from a reservoir through a pipe, the quantity which passes any point in the pipe in one second (current strength) depends on the height of the reservoir above the outlet, that is,

on the "head" or pressure under which it flows, and also on the resistance which the pipe offers to its motion. The greater the pressure the greater the flow, and the greater the resistance the less the flow. The strength of the current is, therefore, directly proportional to the pressure, and inversely proportional to the resistance. If, in this statement, "electro-motive force " be substituted for "pressure," it becomes Ohm's law. When these elements are measured in the units given above, the electro-motive force in volts, the resistance in ohms, and the current in amperes, the law is expressed very simply by saying that the "current is equal to the electro-motive force divided by the resistance."

Thus, if the electro-motive force be one volt, and the resistance of the circuit be one ohm, the current will be one ampere. In an ordinary incandescent electric lamp, the electro-motive force may be about one hundred and ten volts, the resistance of.the carbon filament when hot about one hundred and seventy-five ohms, and the current must therefore be about six tenths of an ampere.

The unit of quantity is the coulomb. Charles Augustus Coulomb was a French engineer who made important contributions to science during the latter half of the last century. His character is well shown by the fact that he submitted to imprisonment rather than make a favorable report upon a proposed system of canals which he examined as a royal commissioner, and which he could not approve. His ingenious invention, the torsion balance, enabled him to measure exceedingly small forces with an accuracy hitherto unknown in science; and by its use he made many brilliant researches in electricity, the first in which exact measurement played an important part. A coulomb is the quantity of electricity transferred by a current of one ampere in one second.

The unit of capacity is the farad. The name of Faraday might with proNO. 439. 39

VOL. LXXIII.

priety have attached to more than one unit of electrical measure. His remarkable career, as a newsboy, a bookbinder's apprentice, an intensely interested listener to the lectures of Sir Humphry Davy, Davy's helper, later his assistant, and finally his successor at the head of the Royal Institution in London, is so generally known that reference to it is hardly necessary. In the history of electricity, three splendid discoveries. stand incomparably above all others. With the first the names of Galvani and Volta are associated, in the discovery of the new electricity and the means of generating it. In the second, Oersted and Ampère united in laying the foundation of the science of electro-magnetism. The third was the discovery of "induction," in which Faraday and Joseph Henry made possible the marvelous development of the last two decades.

But every branch of the subject was enriched by Faraday, and among his most brilliant investigations are those relating to the "capacity of condensers," and especially the influence of the dielectric.

If an insulated conductor is charged with electricity, the quantity which exists upon it will depend on the potential and the capacity of the conductor. It is exactly as if one spoke of the quantity of water in a lake or pond as depending on the depth (pressure, or " potential ") and the area of the bottom (capacity). If two conductors are near each other, but separated by a comparatively thin layer of air, glass, shellac, or other dielectric, the "capacity" of the combination is much greater than that of either of the conductors, and it is known as a "condenser." The well-known Leyden jar is a common type. In speaking of the "potential" to which a condenser is charged, the word is used very much in the same sense as "electro-motive force" in what has gone before. Potential, therefore, may be, and constantly is, expressed in volts. The unit of capacity, the farad,

is the capacity of a condenser which is charged to a potential of one volt by one coulomb of electricity.

To continue the analogy already used, the unit of capacity (area of bottom) for vessels holding water might be defined. as that which would require unit depth to hold unit quantity.

The joule is the unit of work. The name of James Prescott Joule will forever be associated with the most splendid generalization of the present age, namely, the principle of the conservation of energy. Through his interest in electromagnetism, and especially by his investigation of the efficiency of electric motors, he was led to the consideration of the correlation of the various forces of nature, and associated with Professor William Thomson, now Lord Kelvin, he executed a remarkable series of experiments affording cumulative proof of the indestructibility of energy. With With great appropriateness his name has been given to the unit of work. It is related directly and simply to the "erg," which is the unit of work of the centimetregramme-second system. Reference has already been made to the fact that when a current of electricity is passed through a conductor heat is generated, the amount depending on the resistance of the conductor and the strength of the This heat is the equivalent of the energy electrically expended. The joule is the energy expended in one second by a current of one ampere passing through a resistance of one ohm. In the common incandescent or glow lamp, the energy expended as heat in the carbon filament is about 63 joules in every second.

current.

In addition to the unit of work, it is also extremely desirable to have a unit of rate of work, or, as it has been called by many writers, "activity," but which is more commonly expressed by the word "power." It is only natural that the name of one who was the first to recognize the necessity for a quantitative

evaluation of the rate at which energy was absorbed, and to give numerical expression to it in the definition of the horse power, should be given to the new unit. The watt is also simply related to the centimetre-gramme-second unit of power, and is defined as work done at the rate of one joule per second. The rate of expenditure of energy in the glow lamp already quoted would be 63 watts. One horse power is equal to about 746 watts. When Watt came to Glasgow, he was prevented from securing work as a mathematical - instrument maker by the action of the trades-unions of that time. Fortunately, the door of the great university was opened to him, and there, in the capacity of maker and repairer of instruments and apparatus, his genius received its first encouragement and development. Although by education and training rather a practical than a scientific man, he possessed the true scientific insight to an unusual degree, and is eminently worthy of the associates among whom he is here placed.

The foregoing somewhat lengthy and detailed account of the history and origin of seven of the eight units of electrical measure recently adopted by the International Congress has been thought desirable, if not necessary, to a full understanding of their relation, historically and otherwise, to the eighth and last, the name of which is the title of this article.

Most Americans are more or less familiar with the name and fame of Joseph Henry. To many he is known, however, only as the first secretary of the Smithsonian Institution. Giving a broad and liberal interpretation to the somewhat vague language of the will of its founder, Henry moulded the institution, while it was yet plastic and without traditions, into the form in which it has since essentially existed. He directed its energies into channels very different from those that would have been selected by one whose horizon was narrower than his, and, by steadfastly adhering to his

own splendid conception of its functions as an instrument "for the increase and diffusion of knowledge among men," he made of it an organization which is, and must perpetually be, a benefit and a blessing to all mankind. Others, a smaller number, think of him as the youthful professor of mathematics and natural philosophy in the Albany Academy, where, in spite of the seven solid hours of teaching each day required of him, he found time to begin the series of researches in electro-magnetism which in later years were to make him famous. Here, and at the College of New Jersey at Princeton, to which he was shortly transferred, he is seen pursuing these researches with that clearness of vision which characterized his work along all lines, and with an extraordinary fruitfulness which goes only with great intellectual activity accompanied by unflinching honesty of purpose. For fourteen years at Princeton, where he discharged the duties of professor of natural philosophy with signal success, he continued his original investigations, which, while touching many of the more important branches of physical science, were in general related to his favorite subjects, electricity and magnetism. At the end of this period, when at the very highest development of his powers, he was transferred to that larger field of activity and usefulness which was offered by the new institution at Washington, to enter which he knowingly, and against the wishes of many of his friends, abandoned the practically assured prospect of lasting fame as one of the three or four most distinguished physicists of the present century. During these years the work was done which justifies and demands the recognition accorded to it in bestowing upon Henry the high honor of a place in the galaxy of famous physicists whose names will be perpetuated in the metrological nomenclature of all modern languages. In much of this work he was running on

often

lines parallel to those followed by an English philosopher who is doubtless justly entitled to be considered as the first experimental physicist of the present age. Although older by several years than Henry, Faraday began his series of memorable investigations in electricity about the time Henry presented his first papers on the same subject before the Albany Institute, a local scientific society of which he was a member. From this time forward they were "treading upon each other's heels." In the early thirties great scientific discoveries were not announced in all parts of the world within twenty-four hours of their making, as is done to-day, thanks to the labor of these same two philosophers, who, sixty years ago, owing to infrequent communication across the sea and scanty means of publication on either side, were often ignorant of an important advance for some years after it had been made. Henry's innate modesty made him slow to recognize, at least to acknowledge, the value of what he did, and there is no doubt that he lost much in the way of general recognition by his failure to bring the results of his investigations promptly to the attention of the scientific public. Indeed, it was sometimes the urgency of his friends, more jealous than himself of his scientific reputation, that secured the tardy publication of important papers. At that date, far removed both in space and time from the centre of scientific activity, he often contended with the discouraging yet natural and almost necessary fact that some of his finest work had been anticipated by those who had the start of him in time, and the advantage in facilities and re

sources.

On August 30, 1831, Faraday made his splendid discovery of electro-magnetic induction. Before this time Henry had investigated the conditions necessary to the production of a strong magnetic field, and had constructed by far the most powerful magnet known up to

that day. Ignorant of Faraday's work, he planned and began in August, 1831, a series of experiments with a still more powerful magnet, having in view the discovery of a method of producing electricity from magnetism which Faraday was then on the eve of making. But, as already stated, his duties in the Academy were exacting, and, being interrupted, he was prevented from returning to the subject for nearly a year. In the mean time news of Faraday's discovery had crossed the ocean, a meagre account of his results having reached Henry some time early in the summer of 1832. He at once took up the subject, and by the aid of his powerful apparatus was enabled to produce striking verifications and extensions of Faraday's conclusions. A description of these experiments was published in Silliman's American Journal of Science for July, 1832, and the article contains the first announcement of a most important discovery, in which he anticipated Faraday by several years. "Ik Marvel" wrote a sentence in Dream Life, which has been an inspiration to many a young man, “There is no genius in life like the genius of energy and industry;" and if the genius is to develop in the direction of experimental science he might well have added, "and the genius of attention to apparently unimportant, accidental phenomena." It was this trait that was so highly developed in his character, this anxious solicitude that nothing, however trivial it might at the time seem, should escape without note, that brought to Henry the honor of the discovery of self-induction.

Faraday had found that when a current of electricity through one circuit was started, or stopped, or altered in strength, a current would be induced in a neighboring circuit; but the induction of a part of the circuit upon another

1 There is good reason for believing that Henry had observed this phenomenon at a much earlier date than that of publication, and

part, or self-induction, had escaped him. Henry saw it in the interesting and previously unobserved fact that if the poles of a battery of no very great power be connected by a long wire, and the circuit be suddenly broken, a spark will be produced at the point of interruption, while if the connecting wire be short a spark will not be produced. He also noted that the effect was increased by coiling the wire into a helix, and he remarked, at the close of the article describing these experiments, “I can account for these phenomena only by supposing the long wire to become charged with electricity, which, by its reaction on itself, projects a spark when the connection is broken.” 1

This was a capital observation, but, although published in 1832, it was apparently unknown to Faraday, who rediscovered the fact a few years later, and announced it as new. As a matter of fact, it appears that Faraday did not himself observe the fundamental phenomenon, but that his attention was called to it by a friend. His announcement was made in the Philosophical Magazine in 1834, and in a communication to the Royal Society in 1835 he extended and enlarged upon the observation.

In much that he had done, however, he had been anticipated by Henry, who, although greatly interrupted in his original investigations by his removal from Albany to Princeton, had himself taken up the phenomenon of self-induction and made an interesting research.

As time and opportunity allowed Henry continued his electrical investigations during the years that followed. He was the first to obtain induction from induced currents, and he made a classic investigation of mutual induction, and of currents of the second, third, fourth, and higher orders. In addition to his discovery of self-induction, his researches that the observation was really made before the discovery of induction by Faraday.