iron can be wound with covered wire. Prof. Joseph Henry, whose electrical researches led to the invention of the telegraph, spent months in wrapping wire with strips of cloth, in order to make the magnets by means of which he showed the possibility of transmitting signals to a distance by electricity. To-day far more powerful magnets than he constructed can be made in an hour.

This wonderful something which we call electricity circulating around coils of covered wire makes an iron core a magnet. The oftener we make it flow around the iron, or, within certain limits, the more turns of wire we put upon our spool, keeping the strength of the electric current constant, the stronger the magnet. Now, in considering this phenomenon we are led to the remarkable fact that a thin covering of silk or cotton can prevent the electric current, such as is used to propel cars, from being conducted from one layer of wire on the spool to the next. The thinnest sheet of paper placed under the trolley of an electric car will stop the car, provided that it is not punctured by the mechanical pressure. Cut a copper wire carrying an electrical current, file the ends square, place a sheet of writing paper between the ends and press them together: the current which was transmitting thousands of horse power is stopped; it is incapable of passing through the insulating substance of the paper. Here we are brought to a realizing sense of one of the chief peculiarities of the method of transmitting power by electricity. No other agency for transmitting power can be stopped by such slight obstacles as electricity. A sheet of writing paper placed across a tube conveying compressed air would be instantly ruptured. It would take a wall of steel at least an inch thick to stand the

pressure of steam which is driving a 10,000-horse-power engine. A thin layer of dirt beneath the wheels of an electric car can prevent the current which propels the car from passing to the rails, and thus back to the power house.

Another striking difference between what we call a steady electrical current in a wire and steam or air at high pressure in a pipe is the absolute stillness which marks its sudden passage from one wire to another one of larger diameter. In the case of air or steam, this sudden passage from a small conductor or pipe to a larger receptacle would be accompanied by a whistle or roar. There is no sound heard in a wire when an electric current is suddenly established in it. One can handle the wire carrying thousands of horse power without experiencing the slightest sensation, and birds can sit on such a wire with the safety that they can rest on a dead limb of a tree. When the wire is broken, however, there is a blinding flash of light and a loud report-a miniature thunderstorm. The only analogy between our pipes conveying air or steam at high pressure and our wire carrying an electrical current is in the heat which we can perceive on both the pipes and the wire. This analogy is a valuable one, which we must bear in mind as we endeavour to discover what electricity is. The transmission of compressed air or of steam by pipes is not affected by the presence or the motion of surrounding objects outside the pipes; a compass needle remains quiescent. The neighbouring pipes are not attracted or repelled from each other. This is not true in the case of the electrical transmission of power. A compass needle instantly points to the wire carrying the current; and iron dust or filings gather around the wire. Two neighbouring wires carrying currents are

attracted to each other if the currents are going in the same direction, or repelled if they are going in opposite directions. The quick removal of one of these wires from its proximity to the other makes the currents in the wires throb, just as a change in the pressure of the air or steam in a pipe would cause a fluctuation in the transmission of power.

The pressure of compressed air or steam is not sensibly affected by a rapid motion of the pipe containing it; for instance, we could quickly revolve a hollow ring like a top about a diameter, allowing steam at high pressure to pass into the ring through a suitable valve at one end of the diameter and out of the ring at the other. The whirling motion of such a moving ring would not sensibly affect the air or steam in the pipe. If a current of electricity, however, should enter and leave a ring made of wire it could be very much affected by the rapid motion of the ring. The quick rolling of a steamship does not modify the pressure of the steam in the pipes conveying it to the engines; it does, however, affect the electric current which is lighting the steamship, although the effect in the case of the comparatively slow motion of the steamship is small. When a loop or ring of copper wire carrying a current is revolved many times a minute before the pole of a powerful magnet a fluctuating effect can be produced which would be sufficient to make the electric lights of the steamship fed by this current blink painfully. The pole of the earth exerts a similar effect on moving wires carrying currents.

The mysterious something which we call an electric current is therefore influenced by the motion of surrounding objects and by the motion of its own path; it has little in common with the compressed air or steam

which is conveying an equal amount of horse power, except in the evidence of heat along its path.

We have said that the only peculiarity which a pipe conveying steam or compressed air possesses in common with a copper wire conveying an electric current resides in the development of heat along the conductor. If we should narrow the bore of a steam pipe to the size of a knitting needle, we should greatly restrict the flow of steam through the pipe and reduce the amount of horse power that was previously transmitted. The same is true of a water pipe: the flow of water is greatly impeded by the constriction of the pipe. In the case of steam and compressed air there is not a notable or great increase of heat when the bore of the conducting pipe is made smaller. With electricity, however, there is a remarkable increase of heat when the conductor is greatly reduced in diameter. This can be seen by examining the electric light in our houses. The main conductors are of large size and of pure copper, while the filament of the lamps is fine, and is of carbon. The narrowing of the electric conductor, therefore, leads to a great development of heat and light; and here we must bear in mind that the only difference between heat and light consists in wave length; the heat waves are much longer than the light waves. As we continue our study we shall find also that the only difference between light, heat, and electricity is in the length of

waves.

Our analogies between pipes conveying horse power by means of steam or compressed air and wires carrying electricity, we have seen, do not lead us far. There is an evidence of pressure in such pipes, and also on conductors carrying electric currents. There is a development of heat on both pipes and on electric conductors,

but this development is much greater in the case of electricity. Here our analogy stops short. The steam pipe exerts no influence outside itself to attract or repel other pipes. Its effect on a magnet is the same whether the steam flows through it or not; it acts in both cases like a piece of iron; it is not aroused to the exertion of a tremendous power on a neighbouring magnet or on a neighbouring conductor carrying a current when steam rushes through it; it is dead to changing magnetic influences. But the electrical current in a conductor seems to exert a mysterious influence on all neighbouring objects, even on the surrounding air. This influence is especially marked on magnets and on neighbouring electric currents. The western end of the Jefferson Physical Laboratory of Harvard University was constructed with especial reference to freedom from magnetic disturbances, in order that delicate experiments in electricity and magnetism might be conducted there. All the steam pipes and gas pipes were constructed of brass, and no iron nails were used in the flooring. This construction was costly; but it has been made useless by the presence of the overhead wire of an electric road. Every time an electric car passes a fluctuation of the electric current is caused on the overhead wire; all the delicate magnetic instruments in the laboratory throb in unison with the electric disturbance, and this, too, at a distance of at least 400 feet. In scientifically enlightened Germany the electric cars are not permitted to run near the great physical laboratory at Charlottenburg, near Berlin. We see, therefore, that the mysterious phenomenon we call an electric current is extremely versatile. It more nearly resembles the nervous action of man than any other influence with which we have to deal. We can not, however, speak of a nerve current, and we