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:"—

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