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the wire connecting the dynamo at the central station with the motor in the electric car. It may be said to converge upon the motor in the car, and is there reconverted into motion. The flow of current, or the rate of electrification along the wire, is not in the same direction as the flow of electrical energy in the space not filled with the wire. We see here that the water analogies do not aid us, for the water pumped in at one end of a pipe transmits along the interior of the pipe the energy that is exerted at the transmitting end. None of the energy of the transmitter which in this case is the pump is transmitted through the space outside the pipe.

CHAPTER VII.

THE GALVANOMETER.

OUR general survey of the phenomena of the electric current presents the great main features of the subject of electricity as it was known to the world in 1830, if we except the phenomenon of the spreading out of the electric current in the earth and the use of the earth as a return circuit. The knowledge of the action of a battery in the early part of this century strongly resembles the state of the world's knowledge of the action of the human heart in 1630. Two hundred years previous to 1830 Harvey had shown that the arterial blood flowed out from the heart, and, being converted into venous blood, flowed back again. The heart of men and animals was, like the battery, the mysterious source of a strange circulation which was followed only imperfectly by Harvey, for he needed finer and more subtle means of tracing the entire extent of this circulation through the minute vessels which are called capillaries. Of one thing, at least, he was certain the source of the circulation was the action of the heart, and in the dedication of his book on the circulation of the blood to Charles I he reminds this illustrious prince that, "as the heart of animals is the foundation of their life, the source of everything within them, the sun of their microcosm, that upon which all

growth depends, from whom all power proceeds, the king in like manner is the foundation of his kingdom, the sun of the world around him, the heart of his republic, the fountain whence all power, all

flow."

grace doth

It was not until 1690 that Leuwenhoek showed by means of the microscope that "the blood passes from the arteries into the veins by a network of minute vessels, the thin walls of which allow the fluid plasma to transude into the tissues so as to serve for their nutrition."

When Harvey explained his theory of the circulation of the blood to Charles I, the king must have regarded the heart much as the educated man to-day, ignorant of electricity, regards the power house from which the electrical current apparently proceeds and returns through the bosom of the earth, he knows not how. Indeed, Harvey, although he was certain of the main facts, was at a loss to explain how the blood got from the arteries to the veins. A delicate instrument was needed to help the human eye, and in the hands of Leuwenhoek the microscope proved to be this instrument. This little instrument has greatly extended our knowledge of the immense activities which result from the action of the heart. When, too, Volta was summoned to Paris to explain the electric battery to the great Napoleon, the emperor must have regarded the voltaic cell as a heart, from which a mysterious flow proceeded, no one knew why or how. Volta himself probably never suspected that pulsations in the action of the cell could be detected by an electrical microscope in all neighbouring masses of metal, and indeed in space itself. This electrical microscope is termed a galvanometer, and it plays a part in electrical science very simi

lar to that enacted by the microscope in medical science. The microscope and the galvanometer illustrate what immense advances in our knowledge can be made by suitable instruments. By means of the galvanometer, Michael Faraday and Joseph Henry discovered the principle of the dynamo machine.

It is a strange reflection that these philosophers could have made their discoveries by merely employing the microscope, using the agency of light to discover the manifestations of electricity. We have said that a compass instantly points to a wire through which a current of electricity is passing, and that fine magnetic filings cling to the wire. By distributing iron dust on the stage of a microscope near a wire through which a current of electricity is made to pulsate, an observer looking at the particles of dust through the microscope will see them pulsate also. Furthermore, by winding two little bobbins with fine insulated wire, or, in other words, making little wire spools, similar to small spools of thread, connecting the two ends of the wire on one spool to the ends of the wire on the other spool, placing one spool on the stage of the microscope with iron filings on a piece of paper laid on its end, and placing the other spool on a third spool made of coarse wire the ends of which are connected with a battery, one can observe that when the battery current is suddenly made or suddenly broken the little particles of iron vibrate, showing the existence of currents of electricity in the circuit of wire on the two connected spools. This is the great discovery of currents of induction, made by Henry and Faraday, and it is the foundation of the action of the dynamo and of the telephone. It shows that any change in an electric current on a wire, any pulsation, causes instantly a similar pulsation in any neighbouring

wire not connected with the first wire and placed parallel to it.

and

One of the principal objections to using the microscope in this manner resides in the friction between the fine particles of iron which tends to prevent their free motion. A better way is to suspend a very fine cambric needle from its middle by a spider thread or a bit of coccoon fibre drawn from a white silk thread. Make the needle a magnet by allowing it to rest for a moment across the poles of a strong horseshoe magnet; place the little bobbin of wire not on its end, but on its side, on the stage of the microscope, and bring one end of the needle near the end of the bobbin and focus the microscope upon this end. The needle should be protected from currents of air. It will be a compass pointing north and south and the ends of the bobbin should lie east and west. This microscopic galvanometer can be made to detect very feeble currents of electricity, will show that any change of strength of an electric current in the bobbin connected with the battery will manifest itself in a neighbouring bobbin in a circuit of wire totally disconnected and independent of the battery circuit. Furthermore, the extent of movement of the needle over a suitably graduated scale of fine divisions will measure the electrical strength of the impulses given to the needle. The microscope thus magnifies the motion of the magnet. Henry and Faraday, however, were ignorant of the use of a microscope for the detection of the very feeble currents which afterward were exalted into the tremendous currents which drive our electric cars and light our cities, and used the unassisted eye to observe the motion of a fine magnetized cambric needle, one pole of which was opposite the end of a bobbin or spool covered with many turns of very

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