Page images
PDF
EPUB

variations of the magnet are recorded on a continuous register. The means employed consists in throwing a beam of light from a lamp on to a light mirror attached to the magnet whose motion is to be observed. A spot of light is thus reflected upon a ribbon of photographic paper prepared so as to be sensitive to light. The paper is moved continuously forward by a clockwork train; and if the magnet be at rest the dark trace on the paper will be simply a straight line. If, however, the magnet moves aside, the spot of light reflected from the mirror will be displaced, and the photographed line will be curved or crooked. Comparison of such records, or magnetographs, from stations widely apart on the earth's surface, promises to afford much light upon the cause of the changes of the earth's magnetism, to which hitherto no reliable origin has been with certainty assigned. Schuster has shown that these changes generally come from without, and not from within.

161. Theory of Earth's Magnetism. The phenomenon of earth-currents (Art. 233) appears to be connected with that of the changes in the earth's magnetism, and can be observed whenever there is a display of aurora, and during a magnetic storm; but it is not yet determined whether these currents are due to the variations in the magnetism of the earth, or whether these variations are due to the currents. It is known that the evaporation (see Art. 71) always going on in the tropics causes the ascending currents of heated air to be electrified positively relatively to the earth. These air-currents travel northward and southward toward the colder polar regions, where they descend. These streams of electrified air will act (see Art. 397) like true electric currents, and as the earth rotates within them it will be acted upon magnetically. The author has for twelve years upheld the view that this thermodynamic production of polar currents in conjunction with the earth's diurnal rotation affords the only rational means yet suggested

L

for accounting for the growth of the earth's magnetism to its present state. The action of the sun and moon in raising tides in the atmosphere might account for the variations mentioned in Art. 155. It is important to note that in all magnetic storms the intensity of the perturbations is greatest in the regions nearest the poles; also, that the magnetic poles coincide very nearly with the regions of greatest cold; that the region where aurora (Art. 336) are seen in greatest abundance is a region lying nearly symmetrically round the magnetic pole. It may be added that the general direction of the feeble daily earth-currents (Art. 233) is from the poles toward the equator.

[blocks in formation]

162. Flow of Currents. It has been already mentioned, in Lesson IV., how electricity flows away from a charged body through any conducting substance, such as a wire or a wetted string. If, by any arrangement, electricity could be supplied to the body just as fast as it flowed away, a continuous current would be produced. Such a current always flows through a conducting wire, if the ends are kept at different electric potentials. In like manner, a current of heat flows through a rod of metal if the ends are kept at different temperatures, the flow being always from the high temperature to the lower. No exact evidence exists as to the direction in which the current in a wire really "flows." It is convenient to regard the electricity as flowing from positive to negative; or, in other words, the natural direction of an electric current is from the high potential to the low. It is obvious that such a flow tends to bring both to one level of potential. In order that a continuous flow may be kept up there must be a circuit provided. The "current" has sometimes been regarded as a double transfer of positive electricity in one direction, and of negative electricity in the opposite direction. The only evidence to support this very unnecessary supposition

is the fact that, in the decomposition of liquids by the current, some of the elements are liberated at the place where the current enters, others at the place where it leaves the liquid.

The quantity of electricity conveyed by a current is proportional to the current and to the time that it continues to flow. The practical unit of current is called the ampere (see Arts. 207 and 254). The quantity of electricity conveyed by a current of one ampere in one second is called one ampere-second or one coulomb. One amperehour equals 3600 coulombs. If C is the number of amperes of current, t the number of seconds that it lasts, and Q the number of coulombs of electricity thereby conveyed, the relation between them is expressed by the formula:

Q=Cxt.

Example. If a current of 80 amperes flows for 15 minutes the total quantity of electricity conveyed will be 80 × 15 × 60 = 72,000 coulombs. This is equal to 20 ampere-hours.

Currents are called continuous if they flow, without stopping, in one direction. They are called alternate currents if they continually reverse in direction in a regular periodic manner, flowing first in one direction round the circuit and then in the other.

Continuous currents of electricity, such as we have described, are produced by voltaic cells, and batteries of such cells, or else by dynamos driven by power, though there are other sources of currents hereafter to be mentioned. Alternate currents are produced by special alternate current dynamos or alternators, and are separately treated of in Art. 470.

163. Discoveries of Galvani and of Volta. - The discovery of electric currents originated with Galvani, a physician of Bologna, who, about the year 1786, made a series of curious and important observations upon the

convulsive motions produced by the "return-shock" (Art. 29) and other electric discharges upon a frog's leg. He was led by this to the discovery that it was not necessary to use an electric machine to produce these effects, but that a similar convulsive kick was produced in the frog's leg when two dissimilar metals, iron and copper, for example, were placed in contact with a nerve and a muscle respectively, and then brought into contact with each other. Galvani imagined this action to be due to electricity generated by the frog's leg itself. It was, however, proved by Volta, Professor in the University of Pavia, that the electricity arose not from the muscle or nerve, but from the contact of the dissimilar metals. When two metals are placed in contact with one another in the air, one becomes positive and the other negative, as we have seen near the end of Lesson VII., though the charges are very feeble. Volta, however, proved their reality by two different methods.

P

164. The Voltaic Pile. The second of Volta's proofs was less direct, but even more convincing; and consisted in showing that when a number of such contacts of dissimilar metals could be arranged so as to add their electrical effects together, those effects were more powerful in proportion to the number of the contacts. With this view he constructed the apparatus known (in honour of the discoverer) as the Voltaic Pile (Fig. 95). It is made by placing a pair of disks of zinc and copper in contact with one another, then laying on the copper disk a piece of flannel or blotting. paper moistened with brine, then another pair of disks of zinc and copper, and so on, each pair of disks in the pile being separated by a moist conductor. Such a pile, if composed of a number of such pairs of disks, will produce electricity

N

Fig. 95.

« PreviousContinue »