Lightning Conductors.

715

more dangerous than the absence of any protection; and there are many instances of buildings having been set on fire through some fault in the insulation or conductivity of the lightning-rod.

953. It is worthy of remark that, in every kind of weather, there is more or less electricity present in the atmosphere; the quantity, and even the kind of electricity are constantly fluctuating, and the governing laws of these atmospheric changes, are but imperfectly understood.

If a well-insulated lofty metal rod be connected with a common gold-leaf electroscope, the nature and degree of atmospheric electrification overhead can be simply exhibited. Much more sensitive electroscopes have, however, been introduced in recent years, especially the water-dropping collector and others, by Sir W. Thomson. Constant observations with these at Kew Observatory show that the degree of atmospheric clectricity, or the electric potential of the atmosphere, as it is now commonly worded, presents remarkable changes of variation, often within a few minutes.

Its fluctuations, in fact, are almost as variable as those of the wind, and, very probably, are closely connected with them. The following table will give an idea of the extraordinary changes in degree of this atmospheric element :

Average in wet weather or snow (Pɔs. or Neg.) 20 to 30
High wind during frost (Pos.) .

Thunderstorms, frequently (Pos. or Neg.)

80 to 100

100 or more.

200

All observations agree in showing that the average degree of electrification is greater in winter than in summer. But no general theory of the cause of the variations, seems as yet to have been established.*

* According to Professor Daniell, although the atmosphere is not usually so charged with electricity as to produce any marked or visible phenomena, yet it will commonly afford indications of electrical excitement. He found that in calm dry weather, when no clouds were visible, the gold leaves of an electroscope always indicated positive or vitreous electricity, the intensity of which was subject to regular variations reaching a maximum about seven

716

Current Electricity.

CURRENT, GALVANIC, OR VOLTAIC ELECTRICITY.

By the chemical action of an acid on two dissimilar metals, a less demonstrative, but more reliable and serviceable, form of electricity is generated. It is produced in a continuous current, which may be conducted in any desired manner along a metallic path; hence it receives the name of current electricity.

954. It was not until the middle of the last century that the electrical machine was invented, as well as the Leyden jar and the electrical battery, and that Franklin proved the identity of electricity and lightning. The public mind was by these occurrences excited and gratified in the highest degree, and wonder was felt that facts of such vast importance, and apparently not deeply hidden, could have remained unknown so long; but scarcely had this feeling subsided, when a new set of facts connected with electricity drew attention, soon to appear of still greater importance than those above referred to.

Galvani, professor of anatomy at Bologna, having observed that some newly skinned frogs were convulsed when lying near an electrical machine, which he was working, commenced to experiment on the sensibility of dead frogs to the effects of electricity. Happening one day to notice a convulsive motion of the legs of a dead frog, which hung on an iron balcony by a copper hook, he was struck with the resemblance to the twitchings caused by the electric machine. On further examination, he found that when he joined with an iron or copper wire the nerves and muscles in the legs of a frog, the limbs were convulsed every time that the contact was made. He remarked, also, that the convulsion was stronger when different metals, such as iron and copper, were used, one touching the lumbar nerves and the other the muscles of the leg. Each time that contact was completed by connecting the two wires the limbs separated as by a voluntary action.

Galvani ascribed the effect to a source of electricity resident in the animal frame-the nerves having one kind of electricity and the muscles another.

or eight o'clock in the morning, and falling to a minimum between one and two. In high winds and damp weather, without rain, electrical indications can rarely be obtained, and in cloudy weather and during the fall of hail, rain, or snow, they vary much as to kind and intensity.

Volta, another Italian professor, continuing the researches originated by Galvani, came to a quite different conclusion as to the seat of the electricity. He maintained that it was the contact of dissimilar metals or substances which was the exciting cause; and showed that by connecting the muscle with two different metals-iron and copper-the same effects could be produced.

N

P

955. The Voltaic Pile.-To support his theory that the mere contact of different metals was a source of electricity, Volta was led to the construction of what is known as the voltaic pile, shown in fig. 263. Piling a number of zinc and copper discs, separated by moist cloth discs, in the order, copper, zinc, cloth-copper, zinc, cloth-copper, zinc, and connecting the last zinc with the first copper, he obtained much more powerful effects. So marvellous, indeed, were the effects of the combination that it astonished the whole scientific world, and gave an extraordinary stimulus to experimental researches generally.

Fig. 263.

Without attempting to develope the historical growth and progress of the science, we shall merely give the leading results and facts, which is all that the present treatise calls for.

A

956. It is now considered by the highest authorities on electrical science that Volta was so far correct in his hypothesis that the contact of different metals is a source of electricity. If plates of pure zinc and pure copper be placed together they assume opposite electrical states-the one positive, the other negative-to their neutral or normal electrical state. In the language of nodern electricians a difference of electrical potential is set up. Electrical potential is that quality or property in a body by which electricity (whatever elec

Fig. 264.

tricity may be) tends to pass from it to another body, and is measured by the amount of resistance to its passage.

The passage of electricity from a body at a higher potential to one

718

Theory of Voltaic Action.

at a lower, is said to constitute an electric current; so that a lightning flash is to be regarded as a single current of momentary duration passing between bodies whose difference of potential is very great.

In order to produce a continuous current there must, of course, be some means of keeping the two bodies constantly at different potentials; this is nearly accomplished with a Holtz's electric machine. The passage of the electricity between the poles is so rapid as to be nearly continuous, and thus to constitute a sort of current electricity.

Chemical action, however, offers the most simple and easy means of supplying the constancy of different potentials, requisite for the continuous passage of the electricities.

If plates of pure zinc and pure copper be immersed in any acidulated solution, as, for instance, vitriol and water, the zinc and copper assume different electric potentials, just as if in contact; but so long as they are not connected in any way, the tendency of the electricity to pass from the one to the other is prevented. On joining the two plates, however, with a metallic wire, as in fig. 264, the electricity at once passes from the copper, or plate of positive potential, to the zinc or negative plate. This passage is accompanied with a series of very remarkable changes throughout the whole arrangement. If the connecting wire be dipped in iron filings it is found to have acquired magnetic properties; and the water of the solution is decomposed into its constituent elements oxygen and hydrogen. The latter gas bubbles up at the copper plate, and may be ignited with small explosions by applying a match there; the oxygen is liberated at the zinc plate, and by its strong chemical attraction for the latter, enters into immediate union with it. The compound zinc oxide which is formed is instantly dissolved by the acid solution. and so the zinc face is kept clean, and the requisite conditions for a constant difference of potential between the two plates are satisfied.*

* The violent action of acids on commercial zinc is chiefly due to voltaic currents set up by the presence of foreign metals. Pure zinc is scarcely affected by acid of the same strength, and it is with difficulty that hydrogen can be obtained from it. If the bar of pure zinc is wrapped round with platinum wire or foil, a voltaic combination is formed, and hydrogen is evolved. A similar result is obtained with much less trouble by adding a few drops of a solution of sulphate of copper. The copper is deposited on the zinc and hydrogen is at once copiously produced.

Theory of Galvanic Action.

719 Faraday, Davy, De la Rive, Daniell, and many more eminent philosophers, maintained that the real first cause of the electric phenomenon was the chemical action going on. The real explana

tion seems to be that both conditions-metallic difference and chemical action—are indispensable to the production of an electric current. "The flow of electricity is started, as it were, by the difference of potential due to contact; and the continued flow is maintained by the chemical decomposition of the liquid."

D

957. There is in some degree an analogy between electric potential, or electric action generally, and fluid gravity potential, or fluid action in general. Indeed the analogy between the two led to the original fluid theory of electricity; though the modern notions of energy give no countenance to the hypothesis. Water, with a head or potential energy, tends always to flow to a lower level; and compressed gas enclosed in a vessel will, when allowed to communicate with another vessel containing gas, with a less pressure or potential, flow towards the lower potential till there is an equalization of pressure.

A

B

Fig. 265.

Another analogy to the electric current is illustrated by the figure (265). If A and B be two vessels of liquid connected by tubes or pipes, C and D, so as to form a complete circuit full of liquid, then we know, by the laws of heat, as already explained, that on heating the vessel, B, to a temperature greater than that of A, a current will set in, the warmer water rising up through the tube, D, towards the vessel, A, while cold water from A moves along the tube, C, to replace the heated liquid in B. This will continue until all the liquid is equally heated, and the source of heat is powerless to produce any difference of potential in the circuit.

The molecular excitement produced by the heat of the lamp, L, is analogous to that produced by the chemical action of the liquid on the one plate of the galvanic couple; but there is reason to conclude that the analogy is not complete, and that the electric flow is not a material flow along the connecting wire, as we have in the case of this liquid circuit.

Any arrangement, then, for producing electricity in a current, consists generally of a conducting circuit, with a dissimilarity of structure at some part, which is the subject of molecular disturbance;