CHAPTER XIV

ELECTRICITY IN MOTION *

DETECTION OF ELECTRIC CURRENTS

299. Electricity in motion produces a magnetic effect. Let a powerfully charged Leyden jar be discharged through a coil which surrounds an unmagnetized knitting needle, insulated by a glass tube, in the manner shown in Fig. 245, the compass needle being at rest in the position shown. After the discharge the knitting needle will be found to be distinctly magne

tized. If the sign of the charge on the jar is
reversed, the poles of the knitting needle and
the direction of deflection of
the compass needle will in
general be reversed.

The experiment shows that there is a definite connection between electricity and magnetism. Just what

N N

S

FIG. 245. Magnetic effect of an electric current produced from a static charge

this connection is we do not yet know with certainty, but we do know that magnetic effects are always observable near the path of a moving electrical charge, whereas no such effects can ever be observed near a charge at rest.

Let a charged body be brought near a compass needle. It will attract either end of the needle with equal readiness. While the needle is deflected, insert between it and the charge a sheet of zinc, aluminum, brass, or copper. This will act as an electric screen; that is, it will cut off all effect of the charge. The compass needle will at once swing back to its north-and-south position.

*This chapter should be accompanied or, better, preceded by laboratory experiments on the simple cell and on the magnetic effects of a current. See, for example, Experiments 34, 35, 36, 37, and 38 of "Exercises in Laboratory Physics," by Millikan, Gale, and Davis.

Let the compass needle be deflected by a bar magnet, and let the screen be inserted again. The sheet of metal does not cut off the magnetic forces in the slightest degree.

The fact that an electric charge exerts no magnetic force is shown, then, both by the fact that it attracts either end of the compass needle with equal readiness and by the fact that the screen cuts off its action completely, whereas the same screen does not have any effect in cutting off the magnetic force.

An electrical charge in motion is called an electric current, and its presence is most commonly detected by the magnetic effect which it produces. A current of electricity is generally a stream of negative electrons (see Franklin's conception in footnote to § 293, p. 256).

300. The galvanic cell. When a Leyden jar is discharged, only a very small quantity of electricity passes through the connecting wires, since the current lasts for but

FIG. 246. Simple voltaic cell

a small fraction of a second. If we could keep a current flowing continuously through the wire, we should expect the magnetic effect to be much more pronounced. It was in 1786 that Galvani, an Italian anatomist at the University of Bologna, accidentally discovered that there is a chemical method for producing such a continuous current. His discovery was not understood, however, until Volta, while endeavoring to throw light upon it, in 1800 invented an arrangement which is now known sometimes as the voltaic and sometimes as the galvanic cell. This consists, in its simplest form, of a strip of copper and a strip of zinc immersed in dilute sulphuric acid (Fig. 246).

Let the terminals of such a cell be connected for a few seconds to the ends of the coil of Fig. 245 when an unmagnetized needle lies within the glass tube. The needle will be found to have become magnetized much more strongly than before. Again, let the wire which connects the terminals of the cell be held above a magnetic needle, as in Fig. 247; the needle will be strongly deflected.

This page shows in the upper right-hand corner a photograph of the first electromagnet. It was constructed at Princeton in 1828 by Joseph Henry. He wound the arms of a U-shaped piece of iron with several layers of wire insulated by wrapping around it strips of silk. The main illustration is a huge, modern, lifting magnet which itself weighs 8720 pounds, is 5 feet 2 inches in diameter, and can lift a single flat piece of iron weighing 70,000 pounds. It has 118,000 ampere turns, and carries 84 amperes at 220 volts. The coil is built up of several pancakes of copper straps, the turns of strap being insulated from one another by asbestos ribbon wound between them. The magnet is loading a freight car with pig iron, of which its average lift is 4000 pounds

Evidently, then, the wire which connects the terminals of a galvanic cell carries a current of electricity. Historically the second of these experiments, performed by the Danish physicist Oersted (see opposite page 264) in 1820, preceded the discovery of the magnetizing effects of currents upon needles. It created a great deal of excitement at the time, because it was the first clue which had been found to a relationship between electricity and magnetism.

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FIG. 247. Oersted's experiment

301. Plates of a galvanic cell are electrically charged. Since an electric current flows through a wire as soon as it is touched to the zinc and copper strips of a galvanic cell, we at once infer that the terminals of such a cell are electrically charged before they are connected. That this is indeed the case may be shown as follows:

Let a metal plate A (Fig. 248), covered with shellac (a nonconductor) on its lower side and provided with an insulating handle, be placed upon a similar plate B which is in contact with the knob of an electroscope. Let the copper plate of a galvanic cell be connected with A and the zinc plate with B, as in Fig. 248. Then let the connecting wires be removed and the plate A lifted away from B. The opposite electrical charges which were bound by their mutual attractions to the adjacent faces of A and B, so long as these faces were separated only by the thin coat of shellac, are freed as soon as A is lifted, and hence part of the charge on B passes to the leaves of the electroscope. These leaves will be seen to diverge. If an ebonite rod which has been rubbed with flannel or cat's fur is brought near the electroscope, the leaves will diverge still farther, thus showing that the zinc plate of the galvanic cell is negatively charged. If the experi