Frederic Daniell

In the interval between Faraday’s earlier and later papers on the cell, some important results on the same subject were published by Frederic Daniell (b. 1790, d. 1845), Professor of Chemistry in King’s College, London.[47]

Daniell showed that when a current is passed through a solution of a salt in water, the ions which carry the current are those derived from the salt, and not the oxygen and hydrogen ions derived from the water.

This follows since a current divides itself between different mixed electrolytes according to the difficulty of decomposing each, and it is known that pure water can be electrolysed only with great difficulty.

Daniell further showed that the ions arising from (say) sodium sulphate are not represented by Na2O and SO3, but by Na and SO4, and that in such a case as this, sulphuric acid is formed at the anode and soda at the cathode by secondary action, giving rise to the observed evolution of oxygen and hydrogen respectively at these terminals.

The researches of Faraday on the decomposition of chemical compounds placed between electrodes maintained at different potentials led him in 1837 to reflect on the behaviour of such substances as oil of turpentine or sulphur, when placed in the same situation.

These bodies do not conduct electricity, and are not decomposed; but if the metallic faces of a condenser are maintained at a definite potential difference, and if the space between them is occupied by one of these insulating substances, it is found that the charge on either face depends on the nature of the insulating substance.

If for any particular insulator the charge has a value ε times the value which it would have if the intervening body were air, the number ε may be regarded as a measure of the influence which the insulator exerts on the propagation of electrostatic action through it: it was called by Faraday the specific inductive capacity of the insulator.[48]

The discovery of this property of insulating substances or dielectrics raised the question as to whether it could be harmonized with the old ideas of electrostatic action. Consider, for example, the force of attraction or repulsion between two small electrically-charged bodies. So long as they are in air, the force is proportional to the inverse square of the distance; but if the medium in which they are immersed be partly changed—e.g., if a globe of sulphur be inserted in the intervening space—this law is no longer valid: the change in the dielectric affects the distribution of electric intensity throughout the entire field.

The problem could be satisfactorily solved only by forming a physical conception of the action of dielectrics: and such a conception Faraday now put forward.

The original idea had been promulgated long before by his master Davy. Davy, it will be remembered,[49] in his explanation of the voltaic pile, had supposed that at first, before chemical decompositions take place, the liquid plays a part analogous to that of the glass in a Leyden jar, and that in this is involved an electric polarization of the liquid molecules.[50] This hypothesis was now developed by Faraday.

Referring first to his own work on electrolysis, he asserted[51] that the behaviour of a dielectric is exactly the same as that of an electrolyte, up to the point at which the electrolyte breaks down under the electric stress, a dielectric being, in fact, a body which is capable of sustaining the stress without suffering decomposition.

He argued:

“let the electrolyte be water, a plate of ice being coated with platina foil on its two surfaces, and these coatings connected with any continued source of the two electrical powers, the ice will charge like a Leyden arrangement, presenting a case of common induction, but no current will pass. If the ice be liquefied, the induction will now fall to a certain degree, because a current can now pass; but its passing is dependent upon a peculiar molecular arrangement of the particles consistent with the transfer of the elements of the electrolyte in opposite directions As, therefore, in the electrolytic action, induction appeared to be the first step, and decomposition the second (the power of separating these steps from each other by giving the solid or fluid condition to the electrolyte being in our hands); as the induction was the same in its nature as that through air, glass, wax, &c., produced by any of the ordinary means; and as the whole effect in the electrolyte appeared to be an action of the particles thrown into a peculiar or polarized state, I was glad to suspect that common induction itself was in all cases an action of contiguous particles, and that electrical action at a distance (i.e., ordinary inductive action) never occurred except through the influence of the intervening matter.”

Thus at the root of Faraday’s conception of electrostatic induction lay this idea that the whole of the insulating medium through which the action takes place is in a state of polarization similar to that which precedes decomposition in an electrolyte. “Insulators,” he wrote,[53] “may be said to be bodies whose particles can retain the polarized state, whilst conductors are those whose particles cannot be permanently polarized.”

The conception which he at this time entertained of the polarization may be reconstructed from what he had already written concerning electrolytes. He supposed[54] that in the ordinary or unpolarized condition of a body, the molecules consist of atoms which are bound to each other by the forces of chemical affinity, these forces being really electrical in their nature; and that the same forces are exerted, though to a less degree, between atoms which belong to different molecules, thus producing the phenomena of cohesion. When an electric field is set up, a change takes place in the distribution of these forces; some are strengthened and some are weakened, the effect being symmetrical about the direction of the applied electric force.

Such a polarized condition acquired by a dielectric when placed in an electric field presents an evident analogy to the condition of magnetic polarization which is acquired by a mass of soft iron when placed in a magnetic field; and it was therefore natural that in discussing the matter Faraday should introduce lines of electric force, similar to the lines of magnetic force which he had employed so successfully in his previous researches. A line of electric force he defined to be a curve whose tangent at every point has the same direction as the electric intensity.

The changes which take place in an electric field when the dielectric is varied may be very simply described in terms of lines of force. Thus if a mass of sulphur, or other substance of high specific inductive capacity, is introduced into the field, the effect is as if the lines of force tend to crowd into it: as W. Thomson (Kelvin) showed later, they are altered in the same way as the lines of flow of heat, in a case of steady conduction of heat, would be altered by introducing a body of greater conducting power for heat. By studying the figures of the lines of force in a great number of individual cases, Faraday was led to notice that they always dispose themselves as if they were subject to a mutual repulsion, or as if the tubes of force had an inherent tendency to dilate.[55]

It is interesting to interpret by aid of these conceptions the law of Priestley and Coulomb regarding the attraction between two oppositely-charged spheres. In Faraday’s view, the medium intervening between the spheres is the seat of a system of stresses, which may be represented by an attraction or tension along the lines of electric force at every point, together with a mutual repulsion of these lines, or pressure laterally. Where a line of force ends on one of the spheres, its tension is exercised on the sphere: in this way, every surface-element of each sphere is pulled outwards. If the spheres were entirely removed from each other’s influence, the state of stress would be uniform round each sphere, and the pulls on its surface-elements would balance, giving no resultant force on the sphere. But when the two spheres are brought into each other’s presence, the unit lines of force become somewhat more crowded together on the sides of the spheres which face than on the remote sides, and thus the resultant pull on either sphere tends to draw it Loward the other, When the spheres are at distances great compared with their radii, the attraction is nearly proportional to the inverse square of the distance, which is Priestley’s law.

In the following year 1838, Faraday amplified[56] his theory of electrostatic induction, by making further use of the analogy with the induction of magnetism.

Fourteen years previously Poisson had imagined[57] an admirable model of the molecular processes which accompany magnetization; and this was now applied with very little change by Faraday to the case of induc- tion in dielectrics. “The particles of an insulating dielectric,” he suggested,[58] “whilst under induction may be compared to a series of small magnetic needles, or, more correctly still, to a series of small insulated conductors. If the space round a charged globe were filled with a mixture of an insulating dielectric, as oil of turpentine or air, and small globular conductors, as shot, the latter being at a little distance from each other so as to be insulated, then these would in their condition and action exactly resemble what I consider to be the condition and action of the particles of the insulating dielectric itself. If the globe were charged, thcso little con- ductors would all be polar; if the globe were discharged, they would all return to their normal state, to be polarized again upon the recharging of the globe.”

That this explanation accounts for the phenomena of specific inductive capacity may be seen by what follows, which is substantially a translation into electrostatical language of Poisson’s theory of induced magnetism.[59]

Let ρ denote volume-density of clectric charge. For each of Faraday’s “small shot” the integral

integrated throughout the shot, will vanish, since the total charge of the shot is zero: but if r denote the vector (x, y, z), the integral

will not be zero, since it represents the electric polarization of the shot: if there are N shot per unit volume, the quantity

will represent the total polarization per unit volume. If d denote the electric force, and E the average value of d, P will be proportional to E, say

By integration by parts, assuming all the quantities concerned to vary continuously and to vanish at infinity, we have

where φ denotes an arbitrary function, and the volume-integrals are taken throughout infinite space. This equation shows that the polar-distribution of electric charge on the shot is equivalent to a volume-distribution throughout space, of density

Now the fundamental equation of electrostatics may in suitable units be written,

and this gives on averaging

where ρ1 denotes the volume-density of free electric charge, i.e. excluding that in the doublets; or

or

This is the fundamental equation of electrostatics, as modified in order to take into account the effect of the specific inductive capacity ε.

The conception of action propagated step by step through a medium by the influence of contiguous particles had a firm hold on Faraday’s mind, and was applied by him in almost every part of physics. “It appears to me possible,” he wrote in 1838,[60] “and even probable, that magnetic action may be communicated to a distance by the action of the intervening particles, in a manner having a relation to the way in which the inductive forces of static electricity are transferred to a distance, the intervening particles assuming for the time more or less of a peculiar condition, which (though with a very imperfect idea) I have several times expressed by the term electro-tonic state."[61]

The same set of ideas sufficed to explain electric currents. Conduction, Faraday suggested,[62] might be “an action of contiguous particles, dependent on the forces developed in electrical excitement; these forces bring the particles into a state of tension or polarity;[63] and being in this state the contiguous particles have a power or capability of communicating these forces, one to the other, by which they are lowered and discharge occurs.”

After working strenuously for the ten years which followed the discovery of induced currents, Faraday found in 1841 that his health was affected; and for four years he rested. A second period of brilliant discoveries began in 1845.

Many experiments had been made at different times by various investigators[64] with the purpose of discovering a connexion between magnetism and light. These had generally taken the form of attempts to magnetize bodies by exposure in particular ways to particular kinds of radiation; and a successful issue had been more than once reported, only to be negatived on re-examination.