are connected with the terminals of wire wound on a similar spool, a little magnet provided with a mirror, and suspended opposite the end of the latter spool, will show a momentary electric current in the electric circuit constituted by the two connected bobbins. It dies out immediately, but it is instantly renewed when we pull out the pole of the magnet instead of thrusting it into the bobbin. When we pull the pole out, the suspended magnet moves in one way, and when we thrust it in, it moves in an opposite way. Furthermore, if we thrust the south pole of a magnet into the bobbin the galvanometer needle moves in one direction, and when we thrust in the north pole it moves in another direction. Since we perceive that any movement of a magnet near a coil of wire is attended by an electrical disturbance in the wire, it is natural to suppose that we can produce electrical disturbances by keeping the magnet fixed in position and by moving the coil of wire. Experiment will speedily verify this conclusion. We find that a movement of our little bobbin or coil of wire in the air near a pole of a magnet excites currents in the circuit connected with the bobbin.

Since the earth appears to be a magnet, we should expect, therefore, to find that if we move our little coil in the air we should obtain an electrical current. Experiment shows this to be true, and it still further illustrates what the delicate galvanometer revealed to Faraday and Henry. There was something in the space outside a magnet which could be made evident by moving either a wire or the magnet. The slightest change in the current passing through one wire makes itself manifest across space in a distant wire. The galvanometer can detect slight molecular disturbance; it can also reveal mysterious effects in the ether of space. One

sometimes smiles at the microscopist who disputes with a fellow-microscopist on the possibility of measuring spaces of one hundred thousandth of an inch. Yet one should reflect that the antiseptic treatment in surgery, which saves thousands of lives every year, is due to the perfection of the microscope. To those unlearned in electrical science, too, the tiny movement of a needle or the excursion of a spot of light over a scale seems hardly worthy the attention of a liberally educated man. Yet the modern dynamo and the telephone owe their existence to the study by Faraday and Henry of these minute movements.

I have endeavoured to explain the construction of the galvanometer, which I have termed the electrical microscope, and in the following chapter I shall endeavour by its aid to explain the action of the dynamo machine and the electrical motor which is now used to propel electric cars. The galvanometer, we have seen, consists in its essentials of a coil or bobbin of wire like a spool of thread with a tiny magnet hung by a spider thread near one end of the spool. When a current of electricity flows through the spool it makes the spool an electro-magnet with two poles, and the pole near one end of the spool consequently attracts one pole of the little suspended magnet and repels the other, thus causing the tiny magnet to turn around its axis of suspension. When the current ceases the suspended magnet returns to its original position, pointing north and south. The amount of its turn measures the strength of the electro-magnet.

There is another instrument called the electrometer, which is used by electricians to measure the electromotive force instead of the current. The galvanometer indicates the current which results from a certain electro

motive force; it does not measure directly the electromotive force which gives rise to the current. For instance, it will measure the current given by a silver spoon and an iron spoon immersed in a tumbler of water, but it does not tell us directly what is the electro-motive force between the iron and the silver. This electromotive force is similar to the pressure of the water or the steam at the power house, under which pressure the circulation is caused in the pipes issuing from the power house. It has been found that any two metals immersed in a liquid which acts unequally upon them give rise to an electro-motive force, which varies in strength with the character of the metals. It is therefore useful to have an instrument which will serve as a sort of gauge of electrical pressure. This instrument is called a voltmeter, the volt being the unit of electro-motive force. The indications of this instrument are graduated by means of what is called the Latimer Clark cell. This is a voltaic cell which produces a constant electro-motive force and which therefore serves as a unit.

CHAPTER VIII.

THE DYNAMO MACHINE.

WE have remarked that the ancients could not have possessed dynamo machines or telephones, for they were ignorant of the art of covering wire by cotton or silk, and it is doubtful whether they knew the art of drawing wire. If by any cataclysm or widespread catastrophe the European nations should disappear from the face of the earth and some East Indian tribe ignorant of electricity should alone survive, and through the slow ages rise and possess the American continent, their archæologists might find the ancient ruins filled with copper wires. There is no trace of such wires in the ruins of Egypt or of Greece. In reading the Life of Faraday, and his account of his discovery of the principle underlying the great advances in electricity, we wonder why the phenomena which he discovered had not been discovered before, and by men with less mental equipment. When we consider, however, the form of galvanometer with which he worked, and the limited supply of insulated copper wire of different degrees of fineness which was at his disposal, our wonder disappears. With a modern galvanometer a senior in Harvard University of fair intelligence could not fail to discover the laws of magnetic induction, for an accidental movement of a magnet near a coil of wire would

certainly reveal these laws. Faraday's merit consisted not so much in the discovery of the phenomena as in his mental conception of lines of force pervading all space.

It will be interesting, therefore, to compare Faraday's instruments with the more sensitive modern apparatus. In the first place, in common with Tyndall, he used a form of needle galvanometer, and he observed the movements of the ends of the needles over a graduated circle just as we observe in a pocket compass the movement of the needle of the compass over the divisions of the circle placed beneath the end of the needle. It is evident that a small angular movement of the needle can not be very much magnified on a circle of small dimensions. Faraday's circle was not more than four inches in diameter. In the modern galvanometers what would correspond to circles of ten feet in diameter are often used, thirty times the diameter of Faraday's scale. Let us examine further Faraday's galvanometer and the modern form, remembering that we are now dealing with an electrical microscope which has revealed the great world of electrical activity in which we move.

Faraday's and Henry's galvanometer consisted, in the first place, of two ordinary needles-cambric needles-which were magnetized and fixed one above the other (Fig. 8) to a rigid bar, A B. The poles of one needle were opposed to those of the other-that is, a north pole was placed above a south pole, or a south pole above a north pole. The object of this arrangement was to weaken the effect of the north pole of the earth, so that the arrangement of needles should not be held so strongly to the north and south direction, and should yield to a very slight attracting force at right angles to the direction of the earth's pull on the