great circles of copper wire together to one wire at the north pole and to another wire at the south pole, and lead a current of electricity into the collection of meridian wires at the north pole and out of the collection at the south pole, we should find that very little current would go through any one wire; and if we should connect our telephone wires with two neighbouring wires anywhere along the equator we should hear no click; there is no flow of electricity between points of the same pressure. If, however, we connect one wire of the telephone at one point on the equator and the other wire at a point on the wire globe corresponding to New York, we should hear a click, for there would be a flow between these points.

From such experiments we see that what we call the electric current flows out in all directions from the point where it enters the earth, and appears to converge again to the point where it leaves the ground to enter the wire and to return to the power house or battery. Perhaps the best illustration of the manner in which the electric current spreads out in the earth is afforded by a method of telegraphing without wires, which I described in the Proceedings of the American Academy of Arts and Sciences, and which has lately been repeated by Prof. Rubens in Berlin, and by Mr. Preece of the London telegraphic system. In my paper I remarked: "The theoretical possibility of telegraphing across large bodies

* My original researches were made between the observatory at Cambridge and the city of Boston, which were connected by a timesignal wire. The current upon this wire was broken by a clock at regular intervals. I found that I could hear the clock-beats a mile away from the wire by connecting a telephone to a wire and by grounding the ends of the wire 500 or 600 feet apart and parallel with the time circuit.

of water is evident from this survey which I have undertaken. It is possible to telegraph across the Atlantic Ocean without a cable. Powerful dynamo-electric machines could be placed at some point in Nova Scotia, having one end of their circuit grounded in Florida, with an overhead wire between these points of great conductivity and carefully insulated from the earth except at the two grounds. By exploring the coast of France two points not at the same potential could be found, and by means of a telephone of low resistance the Morse signals sent from Nova Scotia to Florida could be heard in France."

What we have said in regard to the spreading out of the electric effect or current in the earth is entirely applicable to the case of the human body. If one pole of a battery or other source of electricity is applied to the middle of the back and the other pole to the middle of the breast, the electric current which is thus led into the body spreads out like a stream of water through an infinite number of fine holes in a rose jet; it permeates every muscle in a greater or less degree between the back and the breast. Its flow can not be detected in the body by a telephone, but a delicate galvanometer, which is the electrician's microscope, will show its spreading. Not only can the spreading of the current be detected by galvanometers, but this action can also be studied by chemical analysis, as we shall see when we study the passage of electricity through fluids. Before going further in the subject of the earth circuit we can already perceive that the use of the earth for a return is not always desirable, for in the neighbourhood of a great city the earth becomes filled, so to speak, with the electrical flow from the common use of the earth by the telegraph companies. At one time, as I have shown

in the article already referred to, it was possible to adjust one's watch by connecting a telephone to the water pipes and gas pipes in almost any part of Boston and Cambridge, for one could hear the clicks of the observatory clock from which time signals were sent. The telephone companies no longer, however, use the earth for a return circuit on their long-distance lines, and employ an entire metallic circuit of copper wire. This circuit obviates the earth disturbances due to the spreading out of electric circuits; it has also other advantages, which we will study later. It is interesting to observe here that what was once considered a notable practical advantage in telegraphy is fast losing its importance as we refine upon our methods of transmitting intelligence by electricity. We shall also see that the earth is no longer used by the electric light and power companies. We shall see further that what we call the steady current is being replaced by the unsteady current or the to-and-fro current for the electrical transmission of power over great distances; and, still stranger, we shall perceive that there are reasons for believing that there is no electric current or flow of electric energy on the wires which are conveying telegraphic messages or propelling electric cars; and that for very rapid alternating currents copper is really a poor conductor, and glass an excellent one.

There are several terms now in common use in electrical science which serve as measures of value, and I shall endeavour to give a popular explanation of them. The term ampère is used to denote the strength of the electric current; the word volt, to denote the unit of electro-motive force or electrical pressure on the circuit. The current may be said to flow under a head, which is termed the voltage. This head is analogous to a head

of water which forces a current of water through a pipe. The quantity of water which flows through the pipe in a unit of time—say a second-is a measure of the flow of water; the quantity of electricity which flows in a second of time is a measure of the electrical flow, and is called an ampère. This flow meets with a certain electrical resistance, which is termed an ohm. The flow of water, also, through a pipe meets with a resistance in the friction with the pipe. These terms-ampère, volt, and ohm—have passed into daily use. They perpetuate the names of a great Frenchman, a great Italian, and a great German. There are two other terms, not so readily comprehended by fluid analogies: The farad, named for Faraday, the unit of electrical capacity, the unit of electrical quantity we can store up; and the henry, named for the great American Joseph Henry, the unit of inductance or electrical inertia-an inertia which manifests itself when a current suddenly rises or falls. These two terms-farad and henry-have immense importance in the subject of alternating currents of electricity.

CHAPTER VI.

THE VOLTAIC CELL.

Ir is interesting to reflect that the study of electricity received its greatest primal impulses from two nations so unlike in mental characteristics-the AngloSaxon and the Italian. To Benjamin Franklin we owe a clearer conception of the phenomenon of lightning, and to Galvani and Volta is due the discovery of the electrical battery. In later times we are indebted to the Anglo-Saxon for the discovery of the principle underlying the action of the dynamo machine and the telephone. The story of the discovery of the electric battery is well known, but we will repeat it for the sake of pointing out modern interpretations of the mysterious action which puzzled Galvani and Volta. In a small cabinet of the Jefferson Physical Laboratory of Harvard University there are three instruments which represent all that was known about electricity in 1830. There is a Franklin electrical machine of great size, with its glass globe and its rubbers, and its ponderous wheel for turning the globe against the rubbers, ordered by Benjamin Franklin for the College at Cambridge; there is a voltaic battery consisting of a great many zinc plates and copper plates which can be immersed in a suitable acid; finally, there is a large electro-magnet, simply a horseshoe-shaped piece of iron wound with coarse wire. The