CHAPTER III.

MAGNETISM.

To an American, the question What is electricity? has a great national interest, for Count Rumford, Benjamin Franklin, and Joseph Henry have been like electric lights spaced on a dark path of a high mountain, and we are slowly ascending to a summit, having been guided by the rays of their genius. In America, too, the practical applications of electricity have extended with such swiftness that one might say that there is something in the manifold transformations of electricity peculiarly congenial to the American temperament. While the reader, however, may be willing to admit the claims of Benjamin Franklin and of Joseph Henry to be pioneers in the subject of electricity, he may doubt the justice of including Count Rumford with these workers on the question, What is electricity?

Our final answer to this great question will necessarily embrace the labours of Count Rumford in the subject of heat; for his celebrated experiment on the heat developed in the boring of a cannon in Munich drew mens' attention to the transformation of mechanical work into heat, and led them to reflect on the conservation of energy. Guided by this great theory, we have learned that light, heat, electricity, and magnetism can be studied and embraced under one head-that of

the transformations of energy. Electricity no longer stands apart, a mysterious force, as Franklin regarded it, having no connection with light or heat. It is now seen that we can not study it apart from the manifestations of the latter.

Knowing that Count Rumford, when a boy, walked from Woburn to Cambridge, a distance of eight miles, to attend the lectures of Prof. John Winthrop, the first Professor of Physics at Harvard University, I was interested to ascertain how much he learned of the subject of electricity. In the college archives I found a timeworn notebook in the handwriting of Winthrop, and among the notes of excellent lectures on astronomy and a few on light and heat I found, apparently, that but one lecture had been given on magnetism and one on electricity. As a curious illustration of the extent of our knowledge of a great subject less than a century and a half ago, I give the main portion of his notes on the lecture on electricity in 1750: "If a flaxen string be extended and supported, and at one end an excited tube be applied, light bodies will be attracted, and that at the distance of 1,200 feet at the other end. This electricity since the year 1743 has made a considerable noise in the world, upon which it is supposed several of the (at present) hidden phenomena of Nature depend. . . . Men have been so electrized as to have considerable light round their heads and bodies, not unlike the light represented around the heads of saints by the painters."

The entire apparatus to illustrate the subject of electricity and magnetism in Harvard University until the year 1820 consisted merely of two Franklin electrical machines, a collection of Leyden jars, and small apparatus to illustrate the effects of electrical attractions

and repulsions shown by electrified pith balls or similar light objects. I have had the Franklin machine photographed beside a modern electrical machine which can be carried around in the arms, and which has many times the efficiency of the machine employed by Franklin. Fig. 2 shows a cut from this photograph. During the days, therefore, of Prof. Winthrop the knowledge of electrical phenomena was extremely small. It was confined to the observation of the attrac

tion of magnets and of the phenomena of frictional electricity. America, however, in the year 1750 knew as much as Europe, and the physical cabinet of Harvard University was not more poverty-stricken than that of the University of Leyden. Our advance since 1750 has been due to the accurate quantitative investigation of the transformations of energy; and although Benjamin Franklin's brilliant experiment in establishing the identity between the manifestations of lightning

and those of the ordinary electrical machine is often referred to as the beginning of our real knowledge of electricity, I should say that the experiment of Count Rumford in boring the cannon has had far more real influence in the development of true ideas in regard to the transformations of energy in which electricity plays such an important part.

In hitherto unpublished letters of Count Rumford to Pictet, of Geneva, in 1797, now in the possession of the American Academy of Arts and Sciences, Boston, he shows how clearly he had seized upon a fundamental idea of the transformations of energy:

"Your friend Mr. Joly will perhaps have mentioned to you a late experiment of mine in which I caused more than four gallons of water to boil without fire-merely by the heat generated by the friction of two metals rubbed against each other. I have just finished a calculation by which it appears that the heat generated equably, or the stream of heat which flowed with a uniform velocity-if I may so express myself-was, in one

of my experiments, equal to that generated equably in the combustion of nine middling-sized wax candles all burning together or at the same time.

"As the machinery which produced the friction which generated this heat could easily be put and kept in motion by the strength of one strong horse, we see how much heat could be generated by the strength of animals, without either fuel, light, or chemical decomposition.

"I am just now engaged in writing a paper on the subject which I mean to send to the Royal Society. The results of my experiments seem to me to prove to a demonstration that there is no such thing as an igneous fluid, and consequently that caloric has no real exist

ence.

You must not, however, raise your expectations too high respecting my experiments. Though they were made on a large scale and conducted with care, there was nothing very new or very remarkable about them; and as to their results, they prove only this single fact (of which most probably you never had any doubt), that the heat generated by friction is inexhaustible, even when the bodies rubbed together are to all appearance perfectly insulated or put into a situation in which it is evidently impossible for them to receive from the other bodies the heat they are continually giving off.

"It appears to me that that which any insulated body or system of bodies can continue to give off without limitation can not be a material substance. A bell when struck with a hammer gives off sound, but I do not think it would be speaking philosophically to call sound a material substance."

In another letter to Prof. Pictet, dated Paris, May 4, 1804, he says, "I am persuaded that I shall live a sufficiently long time to have the satisfaction of seeing caloric interred with phlogiston in the same tomb."

The science of electricity took an immense stride as soon as the transformations of energy were studied from a mechanical standpoint-in other words, from the standpoint of Count Rumford-and as soon as men abandoned theories of subtle fluids and began to measure the forces of attraction and repulsion and the equivalence between motion and the energy it makes manifest, whether we convert this energy into heat or into electricity. Count Rumford saw clearly only the transformation of mechanical work into heat, and the relation between the work of a horse in producing this transformation and the food which he eats. Indeed, he made a rough calculation of this transformation. We