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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, while 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 now considered to be a stream of negative electrons (see § 293).
299. 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 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 (see opposite p. 240), 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. 242).
FIG. 242. Simple voltaic cell
Let the terminals of such a cell be connected for a few seconds to the ends of the coil of Fig. 241 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. 243; the needle will be strongly deflected.
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 on opposite page) 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.
FIG. 243. Oersted's experiment
300. 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. 244), covered with shellac 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. 244. 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 indeed 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 experiment is repeated with the copper plate in contact with B and the zinc in contact with A, the leaves will be found to be positively charged.
*If the deflection of the gold leaves is too small for purposes of demonstration, let a battery of from five to ten cells be used instead of the single cell. If, however, the plates A and B are three or four inches in diameter, and if their surfaces are very flat, a single cell is sufficient.
This page shows in the upper right-hand corner a photograph of the first electromagnet. It was constructed at Princeton in 1828 by 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
The terminals of a galvanic cell therefore carry positive and negative charges just as do the terminals of an electrical machine in operation. The + charge is always found upon the copper and the -charge upon the zinc. The source of these charges is the chemical action which takes place within the cell. When these terminals are connected by a conductor, a current flows through the latter just as in the case of the electrical machine; and it is the universal custom to consider that it flows from positive to negative (see § 293 and footnote), that is, from copper to zinc. 301. Comparison of a galvanic cell and a static machine. If one of the terminals of a galvanic cell is touched directly to the knob of a gold-leaf electroscope, without the use of the condenser plates A and B of Fig. 244, no divergence of the leaves will be detected; but if one knob of a static machine in operation were so touched, the leaves would probably be torn apart by the violence of the divergence. Since we have seen in § 294 that the divergence of the gold leaves is a measure of the potential of the body to which they are connected, we learn from this experiment that the chemical actions in the galvanic cell are able to produce between its terminals but a very small potential difference in comparison with that produced by the static machine between its terminals. As a matter of fact the potential difference between the terminals of the cell is about one volt, while that between the knobs of the electrical machine may be as much as 200,000 volts.
But if the knobs of the static machine are connected to the ends of the wire of Fig. 243, and the machine operated, the current sent through the wire will not be large enough to produce any appreciable effect upon the needle. Since under these same circumstances the galvanic cell produced a very large effect
FIG. 244. Showing charges on plates of a voltaic cell