magnitude of the difference which exists between the degrees of oxidability of the metals composing the circuit."

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"Now, if the oxidability of a metal is actually related to its voltaic action as stated, it is very evident that the place which a metallic body has in the tension series of the contactists denotes the degree which belongs to the same metal in the scale of oxidability of metallic bodies. Comparing the tension series of the metals obtained by water and the galvanoscope, with the scale of oxidability of the same bodies determined by ordinary chemical methods, it is impossible not to see the great accordance between the two series."

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"Now, since we have a number of electrolytes in which other metaloids than oxygen, such as the haloids, sulphur, and seleneum, play the part of anions in their combination with hydrogen, it follows from what has been said, that the electrical tension series of metals determined with different electrolytes, cannot accord with each other perfectly. This want of accordance has been placed beyond doubt by various experiments, and the number of cases is not very small in which the same two metals manifest a different voltaic relation for each other when they are placed in different electrolytic liquids; so that the same metal which in one liquid is positive towards the second metal, manifests the opposite in another liquid.

"The case of a reversal of voltaic action which the same two metals exhibit in two different liquids must, in accordance with the above statements, always appear when the chemical relation of these metals to the anions of the electrolytes used is not the same; that is, when the affinity of one and the same metal for the two anions of the electrolyte does not exceed the affinity of the other metal for the same anions, or shows the opposite relations."

"Experience above all teaches that in general the proportions of affinity which exist between the metals and oxygen are similar to those which take place between those bodies and the haloids, sulphur, seleneum, &c.; hence the voltaic relations which the metals manifest in electrolytic liquids not containing oxygen, accord so frequently with those which are observed in the same bodies in water." * "Let us now consider those batteries which consist of one metal and two electrolytic liquids.

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"The most interesting example is that composed of water, muriatic acid, and gold.

"This battery yields a current which passes from the gold to the acid, and from this to the water. This current is very weak, and by reason of the rapid positive polarization of the gold immersed in the water, it soon ceases to have a measurable strength.

"The origin of this current depends upon the simple fact, that the gold possesses a greater chemical affinity for the chlorine of the muriatic acid, than for the oxygen of the water."

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"It is easily inferred from the preceding explanation, that all voltaic arrangements consisting of two different electrolytes and a metal must form circuits, in case the metal used has a greater chemical affinity for the anion of one of the electrolytic bodies than for the anion of the other. It is likewise evident that the force of the

current thus produced must be proportional to the difference of the two affinities."

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"It need hardly be mentioned, that other than metallic bodies can also be placed at either end of a continuous series of electrolytic molecules to polarize them. According to the chemical relation which such bodies manifest for the anion or kation of an electrolyte, its molecules will be polarized in the latter or the former direction.

"If, for instance, chlorine be brought in contact with one of the ends of a series of particles of water, the chemical equilibriuta of this molecule will be destroyed, and its hydrogen side will be directed towards the chlorine. If the end of a platinum wire be placed in contact with chlorine, and the other end of the same wire with any particle of water of the same series, a current must arise, passing from this end of the platinum wire through the water to the chlorine, while the latter combines chemically with the hydrogen of the water.

"On the contrary, a non-metallic substance being placed at the end of a continuous series of molecules of water, having a chemical attraction for the anion of this series, polarization of the particles of water will occur, and it will be opposite to that which chlorine occasions in the case mentioned above.

"Such a substance, for instance, is sulphurous acid, which tends to unite with the oxygen of the water. This tendency is sufficient to polarize the particles of water, and under favorable circumstances to set the current in motion.

"By placing at one end of a series of molecules of water, a body which has a chemical affinity for the anions, and at the other end a substance having affinity for the kations of the molecules, it is evident that this series will be under a double polarizing influence, and the electro-motive forces coming into play will mutually increase each other. A series of such electrolytic molecules, having, for instance, chlorine at one of its ends and sulphurous acid at the other, if closed by a conductor forming a voltaic circuit, must generate a current stronger than that which appears in the cases where chlorine alone or sulphurous acid alone are used, other things being the same.

"It is hardly necessary to remark, that my hydrogen and platinum battery, as well as Grove's new gas pile, are voltaic arrangements, which, although presenting some peculiarities, belong to the class of combinations described above."

Schönbein finally describes the so called hyper-oxide battery. By immersing in water a clean platinum plate, and one furnished with a covering of hyper-oxide of lead, a current will arise as soon as the two metal plates are put in metallic connexion; and the positive current will pass from the clean platinum plate, through the liquid to the other covered with the hyper-oxide of lead.

The formation of the current, as well as its direction, is easily explained.

It is well known that half of the oxygen in the hyper-oxide exhibits a great tendency to separate and combine with oxidable bodies, Schönbein has, moreover, shown that this second portion of oxygen in the same hyper-oxide has a greater affinity for oxidable substances

than even uncombined or free oxygen; hence the hyper-oxide will polarize the particles of water in such a manner that the hydrogen sides turn towards the hyper-oxide.

Other hyper-oxides act in like manner.

§ 3. Comparison of the Contact theory with that of Schönbein.—If we compare Schönbein's theory with the contact theory, we must understand that they both run parallel, that the phenomena of the open and closed battery can be explained equally well by both; for Schönbein only removes the place of excitation of electricity from the point of contact of the metals to the point of contact between metal and liquid. But Schönbein's theory has a decided advantage in this-that it can determine beforehand in all voltaic combinations, the direction of the current from the chemical relations of the substance forming the battery, while the contact theory is wanting in such a principle.

That the same metals give a current first in one direction, and then in another, according as one or another liquid is placed between them, is perfectly explicable according to the modified contact theory, from the different electromotive relations of the liquids to the metals. Schönbein's theory not only allows the possibility of a reversal of the current by changing the liquids, but it also tells us in what cases, and why, the currrent is reversed.

Thus Schönbein's theory always determines a priori from the chemical nature of the substances which form the battery, the directions of the current, no matter whether the battery is formed of two metals. and a liquid, or of two liquids and a metal; while, on the contrary, the contact theory in many cases is so much at fault that it is unable to determine beforehand the direction of the current from a general principle, and in such cases (e. g. in batteries of water, muriatic acid and gold; water, sulphurous acid and platinum,) an experiment is required to find the direction of the current.

From these considerations, one would suppose that there could be no doubt as to which of the two theories should prevail; whether Schönbein's chemical theory, or the modified contact theory. Yet I cannot decide unconditionally for Schönbein's theory, because it entirely ignores a well established fact, the fundamental experiment of Volta, and is unable to give an explanation of it.

That electricity is generated by different metals coming in contact with each other, is a fact well established by experiments, purposely instituted in various forms, and which cannot be ignored nor set aside by such interpretations of the experiments as the opponents of the contact theory have contrived.

The name contact electricity is exceedingly unfit, and may have contributed not a little to the confusion of the discussion in question; properly speaking, all electricity, wherever and however it may appear, is contact electricity; for, in generating electricity, two different kinds of bodies are necessarily, under all circumstances, brought into contact. In electrical machines, glass and amalgam; in the voltaic pile, two metals and a liquid; in the thermo pile, different metallic rods. Wherever heterogeneous substances are brought into contact, a development of electricity takes place, but generally a state of elec

trical equilibrium soon ensue. For a continuous excitation of electricity this state of equilibrium must be continuously destroyed; this is done in frictional electricity, by removing the contact of the closelytouching places of the heterogeneous substances; in the hydro battery, by the decomposition of the electrolytes; in the thermo pile, the circulation of electrical equilibrium is produced by the disturbance of thermal equilibrium.

SECTION SECOND.

DETERMINATION OF THE CONSTANT VOLTAIC BATTERY.

§ 4. Unit of force of current.-Every conductor of electricity, however good, opposes some resistance to its propagation, and many researches have been made to determine the laws of the transfer through conducting media. The following facts have been established by experiment:

1. Galvanic electricity tends to diffuse itself through the whole capacity of a conductor, and consequently the resistance to conduction will be in proportion inversely to the transverse section of a conductor.

2. All parts of a closed circuit, including the battery itself, are traversed at the same time by the same quantity of electricity, whatever be the diversity of their nature.

It follows from the second law, that the absolute intensity of the electricity that passes in a closed circuit depends upon two circumstances: first, on the force which develops the electricity, and which is called the electro-motive force; and second, on the resistance to conduction presented by the whole circuit taken together. Ohm was the first to give a precise statement of these laws, and to deduce with mathematical precision, from them, consequences which have become of great importance in establishing the theory of the battery as well as in the application of electricity to the arts.

If we designate by S the value of the current, or its power to produce effects, and by E the electro-motive force of a single element, whether this be due to contact chemical action, or both, and by R the resistance in the battery, then the relations may be expressed by the equation

E
S=
R

In the foregoing equation we have supposed that the battery consists of a single element, and that the metals are joined by so short and thick a conductor that it offers no appreciable resistance. If, however, the battery consist of n number of elements, joined as before, then the electro-motive power will be n times greater, and also the resistance will be increased in the same ratio, and therefore we shall have

n E
S=
n R

If, now, we introduce an additional resistance in the conductor

which joins the poles, and represent this by r, then the expression becomes

This is the fundamental equation of Ohm, from which all the relations of galvanic combinations can be derived.

When currents of different forces or strengths are to be compared, there must be, first of all, a common measure. Hitherto, to my knowledge, there have been three different units proposed, each of which we shall consider somewhat in detail.

Pouillet proposed (Pog. Ann., xlii) as a unit of force of the galvanic current that which a thermo-electric element of copper and bismuth would produce, when so closed that the whole resistance is equal to a copper wire of 20 metres long and 1 millimetre thick; one soldering being maintained at a temperature of 100°, the other at 0°.

Fig. 4. Jacobi (Pog. Ann., xlviii 26) compared the deflection of a Nerwander tangent-compass with the decomposition of water produced simultaneously by the current; thus reducing the indications of the tangent-compass to the chemical effect. For unit of force he assumed the current which generates in one minute, one cubic centimetre of explosive gas at the temperature of 0°, and height of the barometer at 760 millimetres.

Weber took for his unit the current which, circulating at a distance around the unit of surface, produced the same action as the unit of free magnetism.

To explain what Weber means by the unit of free magnetism we must dilate somewhat.

A magnetic bar s n placed north or south of a magnetic needle, and perpendicular to the magnetic meridian, as represented in Fig. 4, will tend to deflect the needle from the magnetic meridian, while the terrestrial magnetism tends to draw it back. The magnitude of the deflection depends upon the relation of the two forces; the tangent of the angle of deflection is the quotient of the force of the bar divided by the

denoting by v the angle of deflection, by f the force with which the bar attracts the needle from the magnetic meridian, and by T the force with which the terrestrial magnetism tends to draw it back.

But the action of the bar upon the needle is proportional to the third power of its distance from the needle, so long as this distance is moderately great in comparison with the dimensions of the bar and the needle. Denoting the distance by r, the product fr3 must be a constant quantity, which we will denote by M.

But this product fr3, or M, indicates the moment of revolution which