Part 2
Let N S, Fig. 3, be a bar of hardened steel rendered permanently magnetic. If now there be brought near to it a board-nail, the latter will become a magnet through the _inductive_ action of the first magnet. This induced magnetism may be demonstrated by bringing a tack or other bit of iron to the end that is farthest from the permanent magnet; the tack will adhere to the nail, but will fall off when the nail is removed from the neighborhood of the magnet. By testing the polarity of the nail, it will be found that the end nearest the magnet will be a south pole if the magnet has its north pole towards it, in all cases having a polarity opposite to that of the pole acting upon it. The strength of this induced magnetism thus developed depends upon the distance apart of the magnet and the iron, being at its maximum when the two touch. But the tack itself is also made a magnet, and will attract another tack, and that one still another, the number which can be thus supported being dependent upon the strength of the first or inducing magnet.
Suppose now that we should wind a few feet of wire about the nail, and fasten the two ends of the wire to an ordinary galvanometer, and then make the nail to approach the permanent magnet. The galvanometer needle would be seen to move as the nail approached; and, if the latter were allowed to touch the magnet, the movement of the needle would suddenly be much hastened, but would directly come to rest, showing that, so long as there is no motion of the nail towards or away from the magnet, no electricity is moving in the wire, although the nail is a strong magnet while it is in contact with the permanent magnet. If the nail be now withdrawn, the two phenomena happen as before: that is to say, as the nail recedes it loses its magnetism; and the giving-up of its magnetism induces a current of electricity through the wire in the opposite direction to that it had when the nail approached. The current of electricity in the opposite direction is indicated by the galvanometer needle, which moves according to Ampere's law mentioned on a preceding page.
It may be noted here that we have an effect quite analogous to that already mentioned on page 21 as the experiment of Faraday. In one case a permanent magnet is thrust into a coil of wire, and in the other a piece of iron is made a magnet while enclosed in a coil. In each case there is generated a current of electricity _which lasts no longer than the mechanical motion of the parts lasts_.
MAGNETO-ELECTRIC MACHINES.
Such transient currents are practically useless, and several devices have been invented to make the flow continuous. The common form of machine for doing this may be understood by reference to the diagram.
N S, Fig. 4, is the permanent magnet, which is bent into a U form in order to utilize both poles. N' and S' are short rods of soft iron fastened into a yoke-piece Y, also of soft iron. Coils of wire surround each of the rods as represented, the ends of the wires connecting with each other and with what is called a pole-changer. The whole of this part is capable of revolving upon an axis P Y by a pulley at P. The action is as follows: From their position, the soft-iron rods N' S' must be magnets through the inductive action of the permanent magnet, just as the nail was made a magnet in like position. So long as the parts have the relative position shown in the figure, and there is no motion, no electricity can be developed; but, if the axis P Y be turned, S', which represents the polarity of the rod opposite N, will be losing its induced magnetism; and, when half a revolution has been made, that same pole will be where N' now is; but it will then have N' polarity instead of S'; that is, it has been losing south polarity as it receded from N, and gaining north polarity as it approached S: hence a current of electricity has steadily been flowing through the coil in one direction. At the same time, the other rod N' has passed through similar phases; and its enveloping coil has had a current of electricity induced in it in the same direction as in the first coil. This doubles the intensity of the current; and the whole is conducted by the connecting-wires where the current is wanted. Machines have been built upon this plan, that contained fifty or sixty powerful compound permanent magnets, and as many wire coils, needing a steam-engine of eight or ten horse-power to run them.
A less cumbersome and much more efficient magneto-electric machine has been made by changing the form of the soft iron armature to something like a shuttle, and winding the wire inside of it. This is called the "Siemen's Armature." The latest pattern of such machines is known as the _Gramme_; and its peculiarity consists in the substitution of a broad ring of soft iron for the armature. About this ring a good many coils, of equal lengths, of insulated copper wire are wound in such a manner that one-half of any turn in the wire goes through the inside of the ring, making the coils longitudinal. The whole of the armature thus prepared is fixed upon a shaft, so as to permit rotation, and fixed between the poles of a powerful Jamin magnet. The ends of the coils are connected with conductors upon the axis; and, when the armature thus constructed is rotated, a very constant and powerful current of electricity flows in a single direction, unlike the other forms. It is stated, that, with one-horse power, a light can be obtained equal to that from a battery of fifty Grove cells.
SECONDARY CURRENTS.
So long ago as 1836 it was noticed by Prof. Page of Salem, that, whenever a current of electricity was made to flow in a coil of wire, another current in the opposite direction was induced in a coil that was parallel with the first; and also, when the current in the first was broken, another current in the second coil would flow in the opposite direction to the former one. These currents, which are called secondary currents, are very transient. No current at all flows save at the instant of making or breaking the current. In this respect, we are reminded of the behavior of the soft iron within the coil, which gives origin to a current of electricity when it is made to approach a magnet or recede from it, but gives no current so long as it is still.
These secondary currents were investigated by Prof. Henry, resulting in the discovery of many curious and interesting phenomena. It will be sufficient here for me to refer to what are called induction coils, which are developments of the principles involved in electro-magnetism and electro-induction. Imagine a rod of soft iron of any size to be wound with a coil of wire, the ends of the wire to be so left that they may be connected with a galvanic battery. Around this coil let another coil be wound of very fine and well-insulated wire; the terminal wires of it to be left adjustable to any distance from each other. Now, upon making connection with a battery to the primary coil, there will be two results produced simultaneously. First, the soft iron will be rendered magnetic; and, second, a current of electricity will be generated in the secondary coil; and the strength of this secondary current is very much increased by the inductive action of the soft iron that has been made a magnet. When the battery current is broken, the iron loses its magnetism, and a current of electricity is again started in the secondary coil in the opposite direction. The energy of this derived current is so great that it will jump some distance through the air, and thus is apparently unlike the electricity that originates in a battery. An induction coil made by Mr. Ritchie for the Stevens Institute at Hoboken, N.J., has a primary coil of 195 feet of No. 6 wire. The secondary coil is over fifty miles in length, and is made of No. 36 wire, which is but .005 of an inch in diameter. This instrument has given a spark twenty-one inches in length, with three large cells of a bichromate battery.
Mr. Spottiswood of London has just had completed for him the largest induction coil ever made. It has two primary coils, one containing sixty-seven pounds of wire, and the other eighty-four pounds, the wire being .096 inch in diameter. The secondary coil is two hundred and eighty miles long, and has 381,850 turns. This coil is made in three parts, the diameter of the wire in the first part being .0095 inch; of the second part, .015; and the third part, .011. With five Grove cells this induction coil has given a spark forty-two inches long, and has perforated glass three inches thick.
The electricity thus developed in secondary coils is of the same character as that developed by friction; and all of the experiments usually performed with the latter may be repeated with the former, many of them being greatly heightened in beauty and interest. Such, for instance, are the discharges in vacuo in Geisler tubes, exhibiting stratifications, fluorescence, phosphorescence, the production of ozone in great quantity, decomposition of chemical compounds, &c.
The electricity developed by friction upon glass, wax, resin, and other so-called non-conductors, has heretofore been called static electricity, for the reason that when it was once originated upon a surface it would remain upon it for an indefinite time, or until some conducting body touched it, and thus gave it a way of escape. Thus, a cake of wax if rubbed with a piece of flannel, or struck with a cat-skin or a fox-tail becomes highly electrified, and in a dry atmosphere will remain so for months. Common air has, however, always a notable quantity of moisture in it; and, as water is a conductor of electricity, such damp air moving over the electrified surface will carry off very soon all the electricity.
Again, the electricity developed through chemical action in a battery and through the inter-action of magnets and coils of wire has been called dynamic electricity, inasmuch as it never appeared to exist save when it was in motion in a completed circuit. This, however, is not true; for if one of the wires from a galvanic battery be connected with the earth, and the other wire be attached to a delicate electrometer, it will be found that the latter gives evidence of electrical excitement in the same manner as it does for the electricity developed by friction in another body. This is sometimes called _tension_, and is very slight for a single cell; but in a series of cells it becomes noticeable in other ways. Thus when the terminals of a single cell are taken in the hands, no effect is perceived: if, however, the terminals of a battery consisting of forty or fifty cells be thus taken, a decided shock is felt, not to be compared though with the shock that would be felt from the discharge of a very small Leyden jar. The shock from several hundred cells would be very dangerous.
It was formerly doubted that the electricity would pass between the terminals of a battery without actual contact of the terminals. Gassiot first showed that the spark would jump between the wires of a battery of a large number of cells before actual contact was made. Latterly Mr. De La Rue has been measuring the distance across which the spark would jump, using a battery of a large number of cells.
I give his table as taken from the "Proceedings of the Royal Society:"--
Cells. Striking distance.
600 .0033 inch.
1,200 .0130 "
1,800 .0345 "
2,400 .0535 "
This table shows that the striking distance is very nearly as the square of the number of cells. Thus, with 600 cells the spark jumped .0033 inch; and with double the number of cells, 1,200, the spark jumped .0130 inch, or within .0002 of an inch as far as four times the first distance.
This leads one to ask how big a battery would be needed to give a spark of any given length, say like a flash of lightning. One cell would give a spark .00000001 inch long, and a hundred thousand would give a spark 92 inches long. A million cells would give a spark 764 feet long, a veritable flash of lightning. It is hardly probable that so many as a million cells will ever be made into one connected battery, but it is not improbable that a hundred thousand cells may be. De La Rue has since completed 8,040 cells, and finds that the striking distance of that number is 0.345 inch, a little more than one-third of an inch. He also states that the striking distance increases faster than the above indicated ratio, as determined by experimenting with a still larger number of cells.
These experiments and many others show that there is no essential difference between the so-called static and dynamic electricity. In the one case it is developed upon a surface which has such a molecular character that it cannot be conducted away, every surface molecule being practically a little battery cell with one terminal free in the air, so that when a proper conductor approaches the surface it receives the electricity from millions of cells, and therefore becomes strongly electrified so that a spark may at once be drawn from it.
WHAT IS ELECTRICITY?
THEORIES.
NUMEROUS attempts have been made to explain the phenomena of electricity. As a general thing, these phenomena are so utterly unlike other phenomena that have been explained and are easily intelligible, that it has quite generally been taken for granted, until lately, that something very different from ordinary matter and the laws of forces applicable to it must be involved in the phenomena themselves. Consequently the term _imponderable_ was applied to it,--something that was matter minus some of the essentials of matter; and as it was apparent that, whatever it was, it moved, apparently flowed, from one place to another, the term _fluid_ was applied to it, a term descriptive of a certain form of matter. Imponderable fluid was the descriptive name applied to electricity. Newton supposed that an excited body emitted such a fluid that could penetrate glass. When the two facts of electrical attraction and repulsion had to be accounted for, two theories were propounded,--one by Benjamin Franklin, the other by Dufay. Franklin supposed that electricity was a subtle, imponderable fluid, of which all bodies contained a certain normal quantity. By friction or otherwise this normal quantity was disturbed. If a body received more than its due share, it was said to be positively electrified: if it had less than its normal quantity, it was said to be negatively electrified. Franklin supposed this electric fluid to be highly self-repulsive, and that it powerfully attracted the particles of matter.
According to Dufay, there are two electric fluids, opposite in tendency but equal in amount. When associated together in equal quantities, they neutralize each other completely. A portion of this neutral compound fluid pervades all matter in its unexcited state. By friction or otherwise this compound fluid is decomposed, the rubber and the body rubbed exchanging equal quantities of opposite kinds with each other, leaving one of them positively, the other negatively electrified. These two fluids were supposed to be self-repulsive, but to attract each other: so that, if two bodies be charged with either positive or negative electricity, such bodies would mutually repel each other; but if one was charged with positive, while the other was charged with negative electricity, the two bodies would mutually attract each other.
Either of these two theories may be used to illustrate the phenomena, and so have done good service in systematizing the facts. It is evident that both of them cannot be true, and it is in the highest degree probable that neither of them is true.
Some have supposed that there was a kind of electric atmosphere about every atom of matter; and still another theory, now advocated by Edlund of Stockholm, assumes that electricity is identical with the ether by which radiant energy, light and heat, is transmitted.
Before a correct judgment can be formed of the nature of any force, it is necessary to know what it can do, what kind of phenomena it can produce. Let us, then, take a brief survey of what electricity can do.
1st, It can directly produce _motion_, through the attractions and repulsions of electrified bodies,--as indicated by electrometers, the rotation of the fly-wheel, the deflection of the galvanometer needle. It has been proved by the mathematical labors of Clausius, and confirmed by experiment, that, when electricity performs any mechanical work, so much electricity is lost, annihilated as electricity.
2d, It can directly produce _heat_, as shown by passing a sufficient quantity of electricity through a fine platinum wire: the wire becomes heated, and glows, and it may even be fused by the intensity of the heat. The heat developed in the so-called electric arc is so great as to fuse the most refractory substances. If a current of electricity from a battery be sent through a thermo-pile, one of the faces of the pile will be heated. The heat of the spark from a Leyden jar may be made to ignite gunpowder, and dissipate gold into vapor. The heat produced by lightning is seen when a live tree is struck by a powerful flash: the sap of the tree is instantly converted into steam of so high a tension as to explode the tree, scattering it in small fragments over a wide area. The tips of lightning-rods often exhibit this heating effect, being fused by the passage of too great a quantity of electricity.
In the early part of the present century it was demonstrated by Count Rumford, and also by Sir Humphry Davy, that heat was but a form of molecular motion. Since then the exact relations between the motion of a mass of matter and the equivalent heat have been experimentally determined by Joule, so that the unit of heat may be expressed in the motion of a mass of matter. This is deducible from a more general law, known as the conservation of energy. The application in this place is, that whenever heat appears through electric action, as in the above-mentioned places, we know that it still is only _motion_ that is the product, only that this motion is now among the molecules of the body, instead of the motion of the whole body in space, as when a pith-ball moves, or a galvanometer-needle turns.
3d, It can directly produce _light_. This is seen in every spark from an electric machine, in the flash of lightning, and in the electric light.
It has been shown in numberless ways, that there is no essential difference between light and heat, and that what we call light is only the active relation which certain rays of radiant energy have to the eyes. In order to make this plain, suppose that a beam of light, say from the sun, be permitted to fall upon a triangular prism of glass: at once it is seen that the beam is deflected, and instead of appearing a spot of white light, as it did before it was deflected, it now appears as a brilliant band of colors, which is called the solar spectrum. If now this spectrum be examined as to the distribution of heat, by moving a thermo-pile through it from the blue end towards the red end, it will be noticed that the galvanometer-needle will be but slightly deflected at the blue end; but, as the thermo-pile is moved, the deflections are greater until it is past the red end, where the heat is greatest. On this account it has been customary to say that the red end of the spectrum was the heating end. With various pieces of mechanism the rays may be separated from each other, and measured; and then it appears that a red ray of light has a wave length of about 1/37000 in., and the violet ray about 1/60000 in. The rays beyond the red have also been measured, and found to be greater in length uniformly as one recedes from the visible part of the spectrum.
In like manner, beyond the blue end the wave lengths become shorter and shorter; and in each of these directions the spectrum that is invisible is much longer than the visible one. Now, it has also been found that where a prism of glass or other material is used to produce a spectrum, it distributes the rays very unevenly; that is, towards the red end of the spectrum they are very much crowded, while towards the blue end they are more dispersed. Hence, if one were measuring the heating power of such a spectrum, many more rays would fall upon an equal surface of the thermo-pile at the red end than at the blue end; therefore the indications of the galvanometer would be fallacious. Before any thing definite could be known about the matter, it would plainly be necessary to work with an equal dispersion of all the rays. This was effected a few years ago by Dr. Draper of New York. He took the spectrum produced by diffraction instead of refraction, and measured that. In that way it was found that the heating power of the spectrum is equal in every part of it; and hence the pictures in treatises on physics that represent the heating power of the spectrum to be concentrated at the red end is not true save where the spectrum is irregularly produced. As for vision, the mechanical structure of the eye is such that radiant vibrations having a wave length between 1/37000 in. and 1/60000 in. can affect it, while longer or shorter wave lengths can not. Such waves we call light, but it is not at all improbable that some animals and insects have eyes adapted to either longer or shorter wave-lengths; in which case, what would be perfectly dark to us would be light to them. It is a familiar enough fact, that many animals, such as dogs, cats, rats, and mice, can see in the night. Some horses may be trusted to keep in the road in a dark night, when the driver cannot see even the horse itself. This has usually been accounted for by saying that their eyes are constructed so as to collect a greater number of luminous rays. It is much better explained by supposing their eyes to be constructed to respond to wave-lengths either greater or less than those of mankind.
A ray of light, then, consists of a single line of undulations of a definite wave length, such that if it falls upon the eye it will produce sight; if it falls upon a thermo-pile it heats it by just the same quantity that another wave-length would heat it; if it falls upon matter in unstable chemical relations, it will do chemical work, depending upon the kinds of matter. A red ray is as effective for some substances as a violet ray is for others. The statement, then, so often lately made to do certain analogical work, namely, that a ray of light consists of three distinct parts, which may be separated from each other, and are called heat, light, and chemical properties, is simply untrue. What a ray will do, depends upon what kind of a structure it falls on; and when it has done that work, of whatever kind it may be, it ceases to exist as a ray.
If, therefore, electricity can directly produce light, it is simply producing _motion_, as in the case of heat, the motion being of such a sort that the eyes of men are affected by it.
4th, It can produce _magnetism_. A current of electricity passing through a coil of wire makes such a coil a magnet, which will set itself in the direction of the magnetic meridian of the earth; and, if a bar of soft iron be placed in the coil, it becomes the familiar electro-magnet; and, if hardened steel be put in it, it becomes a permanent magnet.