Capacitance

“ As electricity resides in all substances, and is, perhaps, an essential ingredient in their condition, so every change in the state of matter—whether from heat to cold, or from cold to heat; from a state of rest to that of motion; from the solid to the liquid, or the æriform condition, or vice versa; or whether substances combine chemically and produce new compounds—in every change the electrical equilibrium is disturbed; and, in proportion to the degree of disturbance, is the force exerted by electricity to resume its balance in the scale of nature. ”

“ Electricity tends to flow from places of high to places of low potential, water to flow down hill, and heat to move from places of high to places of low temperature. Returning to the case of the charged body with the space around it cut up into electric cells by the tubes of force and shells of potential, it is obvious that the number of these cells is represented by the product QV, where Q is the charge and V the potential of the body in electrostatic units. ”

“ We have increased their capacity for storing a charge, and for this reason is called the specific inductive capacity of the medium, or also the dielectric constant of the medium. The value of is about for oil, to for paper, for mica, and may go up to as much as for glass. The larger , the greater is the charge with a given force pushing the electricity on to the conductor. This force must, however, not be confounded with the mechanical force of attraction or repulsion with which we have been concerned hitherto ”

“ A unit positive charge, liberated in any point of the field, will follow the impulse of that force and move from point to point along a particular line, and we may thus speak of a "line of force," meaning thereby the path along which a unit charge, or in fact any charge of positive electricity, is urged. ”

“ I have here given you all the illustrations that time will permit me to shew you of chemical affinity producing electricity, and electricity again becoming chemical affinity. Let that suffice for the present, and let us now go a little deeper into the subject of this chemical force, or this electricity—which shall I name first—the one producing the other in a variety of ways? These forces are also wonderful in their power of producing another of the forces we have been considering, namely, that of magnetism ”

“ When a spark of static electricity jumps a long distance, we say that the charge has a high potential; that is, it has a high electrical pressure. Potential, for electricity, means the same as pressure, for water. The greater the potential, or electromotive force (E.M.F.) of a cell, the greater its power to push a current through wires. (See "Study," § 296 to 305, with experiments.) ”

“ I had frequently speculated as to the effect which powerful currents of electricity exercise upon any substance through which they are poured for a considerable time. I did not here mean such feeble currents as are passed along a telegraph wire, but I mean the very highest possible developments. Well, I tried a series of experiments upon this point. I found that in liquids, and in compounds, the force had a disintegrating effect. The well-known experiment of the electrolysis of water will, of course, occur to you. ”

“ To these powers, which are unquestionable and distinct, it is usual to add vital force, of which, however, it is difficult to speak as a whole; but one member of our vital energies, the nerve-force, allied to electricity, fully deserves to rank in the correlation.
Taking the one mechanical force, and those three of the molecular named heat, chemical force, electricity, there has now been established a definite rate of commutation, or exchange, when any one passes into any other. ”

“ What is true for the particular substances which originally led man to discover the dual nature of electricity, holds good equally for any pair of substances capable of assuming the electric state when rubbed against each other. If we examine from this point of view the series of such substances, as usually given in the textbooks on electricity, we shall always find a substance of extreme salt-character at the one end, and one of extreme sulphur-character at the other, the substances as a whole forming a gradual transition from one extreme to the other. ”

“ Now arises the question, Are all these relations between electricity and chemical combination limited to that class of bodies which we know as electrolytes? In order to produce a current of sufficient strength to collect enough of the products of decomposition without producing too much heat in the electrolyte, the substance which we try to decompose ought not to have too much resistance against the current. ”

“ It was found that the quantity of electricity in each discharge is independent of the strength of the current and of the material of the electrodes, and that it depends on the nature and density of the gas, and on the distance and form of the electrodes. ”

“ The most intense concentration of force is displayed by a numerous series of large plates: light and heat are copiously evolved, and chemical decomposition is accomplished with extraordinary energy; whereas the electricity from one pair of plates, whatever their size may be, is so feeble 301that it gives no sign either of attraction or repulsion. Common or static electricity is of greater intensity and has a greater power of overcoming resistance than Voltaic electricity, but it acts upon a smaller quantity of matter. ”

“ Electrical Capacity.—We must next consider the quality of a conductor called its electrical capacity. The potential of a conductor has already been defined as the mechanical work which must be done to bring up a very small body charged with a unit of positive electricity from the earth’s surface or other boundary taken as the place of zero potential to the surface of this conductor in question. ”

“ A dielectric medium, when thus polarized, is the seat of electrical energy, and the energy in unit of volume of the medium is numerically equal to the electric tension on unit of area, both quantities being equal to half the product of the displacement and the resultant electromotive force, or
where is the electric tension, the displacement, the electro motive force, and the specific inductive capacity. ”

“ Again, when two bodies, repellent of each other, act upon a third with different degrees of the same electrical energy, the combination will be determined by the degree; or, if bodies having different degrees of the same electrical energy with respect to a third, have likewise different energies with respect to each other, there may be such a balance of attracting and repelling forces as to produce a triple compound; and by the extension of this reasoning, complicated chemical union may be easily explained. ”

“ Another mode of explanation is given by those who maintain, in opposition to Franklin, that positive and negative electricity are either two distinct fluids, or else vibrations of the same fluid—the positive electricity always rushing out of a body, and the negative always rushing in. Those who maintain this hypothesis endeavour to support it by the easy solution which they affirm it gives to the phenomena of electric attraction and repulsion. ”

“ The relative capacity of a conductor or system to retain a charge of
electricity with the production of a given difference of potential. The
greater the charge for a given change of potential, or the less the
change of potential for a given charge the greater the capacity. The
measure of its capacity is the amount of electricity required to raise
the potential to a stated amount. The unit of capacity is the farad, q.
v. Electric capacity is comparable to the capacity of a bottle for air. ”

“ These enclosing surfaces, therefore, cut up the space into shells of potential, and divide up the tubes of force into electric cells. The surface of a charged conductor is an equipotential surface, because when the electric charge is in equilibrium there is no tendency for electricity to move from one part to the other. ”

“ We need not concern ourselves with the mechanism by which such a process could be carried out, since the whole experiment is only hypothetical and merely intended to illustrate principles. Our unit charge then is a carrier of energy, or a means of storing energy; and the amount of energy stored will depend on the charge on the sphere, the medium in which the approach takes place, and the distance from the centre of the sphere at which the approach is arrested. ”

“ During the passage of unit of electricity through any section of the circuit, one electrochemical equivalent of each of the substances in the cells, whether voltaic or electrolytic, is electrolysed.
The amount of mechanical energy equivalent to any given chemical process can be ascertained by converting the whole energy due to the process into heat, and then expressing the heat in dynamical measure by multiplying the number of thermal units by Joule's mechanical equivalent of heat. ”

“ The charge of a Leyden jar is very exactly proportional to the electromotive force of the charge, that is, to the difference of potentials of the two surfaces, and the charge corresponding to unit of electromotive force is called the capacity of the jar, a constant quantity. The corresponding quantity, which may be called the capacity of the secondary pile, increases when the electromotive force increases. ”

“ My third argument is based upon electric reactions.
Whatever may be the true nature of electricity, it is convenient, and probably leads to no error, to speak of it as a fluid, or rather two fluids. For we find two kinds of electricity, denominated positive and negative; and it is a general fact, that, when a body is brought into one electrical state, it throws other bodies around it into the opposite state, by a power called induction. ”

“ We have seen, in our experiments on the mechanical stimulation of, for example, tin, that a difference of electric potential was induced between the more stimulated and less stimulated parts of the same rod, and that an action current could thus be obtained, on making suitable electrolytic connections. ”

“ In the present state of electrical science, the determination of the electric resistance of a conductor may be considered as the cardinal operation in electricity, in the same sense that the determination of weight is the cardinal operation in chemistry. ”

“ I have lately noticed a striking illustration of the modification of superficial energy by a difference of electrical potential. The experiment can be performed in this way: Fill the lower part of a glass jar with clean mercury, pour a saturated solution of common salt upon the mercury, hang in the salt solution a carbon plate, and connect this plate with a battery of four or five Bunsen cells; and, on connecting an iron wire with the other pole of the battery, touch the surface of the mercury. An amalgam will be speedily formed and chlorine gas given off. ”

“ It is therefore extremely natural to suppose that the currents of the ions are convection currents of electricity, and, in particular, that every molecule of the cation is charged with a certain fixed quantity of positive electricity, which is the same for the molecules of all cations, and that every molecule of the anion is charged with an equal quantity of negative electricity. ”

“ An enquiry of greater importance can scarcely be imagined; involving in its train several of the most abstruse and difficult questions of corpuscular philosophy; as, whether electricity be a subtile fluid, or electrical effect the mere exhibition of the attractive powers of the particles of bodies; for, if it can be shown that these effects take place in a perfect vacuum, we shall advance towards the conclusion, that electrical phenomena depend upon the agency of an ethereal and transcendental fluid. ”

“ Cavallo, proves that the electric matter passes through the substance of the metal, and not over the surface. A wire, adds he, continued through a vacuum is also a convincing proof of this assertion.
123VI. “Positive electricity is an accumulation, or too great a quantity of the electric matter contained in a body; and negative electricity is when there is too little.” This position, like the fourth, must be considered as hypothetical—the peculiar nature of the electric fluid not admitting of experiments to prove, or to disprove it. ”

“ But we soon find that chemical force is developed by various other physical agencies,—by heat, by light, by electricity, by magnetism, by mechanical agencies; and, vice versa, that chemical action develops heat, light, electricity, magnetism, mechanical force, as we see in our matches, galvanic batteries, and explosive compounds. ”

“ In electrical researches accumulators are used for two principal purposes, for receiving and retaining large quantities of electricity in as small a compass as possible, and for measuring definite quantities of electricity by means of the potential to which they raise the accumulator. ”

“ The ions are impelled by the electric force associated with the gradient of potential in the electrolyte. But this is not the only force which acts on them; for, since their available energy decreases as the concentration decreases, there must be a force assisting every process by which the concentration is decreased. ”

“ The effects of capacity are the most striking, for in these experiments, since the self-induction and frequency both are high, the critical capacity is very small, and need be but slightly varied to produce a very considerable change. The experimenter brings his body in contact with the terminals of the secondary of the coil, or attaches to one or both terminals insulated bodies of very small bulk, such as exhausted bulbs, and he produces a considerable rise or fall of potential on the secondary, and greatly affects the flow of the current through the primary coil. ”