by their glass handles; we next apply the trial plane to each of them, at a point similarly situated, for example, upon their circumference, and measure by the torsion balance, the charges thus acquired. They will be proportional to the thickness of the electrical strata at the points of contact, and consequently to the total quantities of electricity of the two plates, since these are supposed equal in magnitude, and the points of contact are similarly situated. Thus the charge taken from the collector plate may represent A, and the charge taken from the lower plate B; and the ratio of the latter to the former will be the ratio of saturation; whence we may deduce by calculation the measure of the condensing force. This method is more certain than to endeavour to determine directly the proportion of condensation, as it would seem that we might do, by comparing with the trial plane the charge which the collector plate receives from the same system of conductors when it is alone and when it is under the influence of the other plate. For, in order that this. comparison may be exact, it is necessary that in the two cases, the conductors should be charged to exactly the same degree; and of this equality we can never be certain.

75. The condensing force being determined, the absolute effect of the condenser depends still on the absolute quantity of electricity which the collector plate would take from the conductors by which it is charged, if it were placed alone in contact with them. But, other things being the same, this quantity must increase with the surface of the collector plate. Therefore condensers of a large diameter will accumulate more electricity than those of a smaller diameter, and must give greater shocks on being discharged; and this is in fact confirmed by experiment.

These reciprocal neutralizations which we have made use of for the purpose of calculation, may be rendered sensible by the following experiment.

76. After charging a condenser constructed with a plate of glass, the lower plate of the condenser communicating with the ground, insulate the whole apparatus, and first touch the lower plate; we shall draw from it no electricity; consequently all the electricity upon it is disguised. Then touch the upper plate, and a spark will be given; still the electricity will not all

be carried off; a considerable portion will remain in a disguised state. To render it sensible, touch anew the lower plate. It will now give a spark; for its electricity is not all disguised, since we have taken away a part of that which retained it by its action at a distance. But by this contact a new portion of the latter has become free; the collector plate will therefore give another spark, and so on till the two plates are completely discharged. It is easy to determine by calculation the law of this progression from the constant ratio of saturation at the distance between the two plates. We thus find that the first contact takes away more electricity than the second; the second more than the third, and so on; and that these quantities follow a decreasing geometrical progression, having for its ratio the ratio of saturation.

When we touch both plates at once, all the electricity which would have escaped from the two faces by the successive contacts, is transmitted simultaneously through the body, and this single shock completely discharges the condenser.

77. I have said above that in the condenser with a piece of glass and naked plates, the greater part of the accumulated electricities does not adhere to the surface of the plates, but attaches itself to the opposite faces of the glass. In that case, the two plates have properly no other effect than to establish a free communication between the different points of each of the two faces of this glass, in order that the electricity may easily spread itself over them and may also escape, at the moment of the discharge, from all their points at once. This may be easily verified by experiment; for this purpose, after having charged such a condenser, place it upon an insulator; then with the hand remove the upper plate by its insulating handle, and touch it; we shall receive from it only a small spark, and the expansive force will remain with the other plate. This being done, remove also the glass plate, lifting it by one of its edges, and touch the lower plate; this will give a spark in its turn, but also very small. It follows from this that the accumulated electricities have remained attached to the two faces of the glass plate; and in fact if we replace it between the two insulated plates of the condenser, without communicating to them, or to it, any new electricity, the condenser will be found to be recharged of itself almost as

strongly as at first. Or otherwise, without replacing the glass between the two plates, if we apply both hands directly to its two faces, so as to touch a great number of points at once, we shall feel a discharge, just as if the glass had again been covered with the plate; because the extent of contact of the hands permits a large number of points of the two surfaces to discharge themselves at once. But if, instead of touching the faces of the glass with the open hands, we merely move over them the extremities of the fingers, we shall only perceive a slight sparkling and a local discharge in the points touched; no general discharge, however, will take place, and thus we shall be exposed to no violent shocks.

78. Epinus, who was indeed the real inventor of this instrument, contrived an experiment in some respects the reverse of the preceding, which shows very evidently what is the precise use of the insulating lamina interposed between the two plates. He employed for plates two large circular pieces of wood covered with sheets of tin; and having brought them toward each other in a parallel direction, without any thing being interposed except the stratum of air which separated them, he caused the upper plate to communicate with the conductors of an electrical machine, the lower communicating with the ground. This apparatus, it will be perceived, is a true condenser, an aerial lamina taking the place of the varnish; it is charged, also, in the same way as a condenser is charged, and it gives a shock when, the lower plate being touched with one hand, the upper is touched with the other. In order to obtain considerable shocks from this apparatus, it is necessary to employ large plates; for since we are obliged to keep them at a considerable distance that sparks may not escape from them directly through the air, the extent of surface must compensate for the weakness of the condensing force. Besides, this extent seems to be one cause which retards the spark when the plates approach parallel to one another. Its effect is in a degree the reverse of the effect of points. The only difference between this and the common condenser is, that the surfaces of the insulating lamina have no real existence, except when the two plates are in presence of each other, for they are nothing else but the aerial limits of the surfaces which the two plates mutually present to each other.

79. Although Epinus actually invented the condenser, as we have said, and gave its true theory, as may be seen in his trea tise, it was Volta, who by uniting it to the electroscope, rendered it useful in discovering and making sensible the most feeble sources of electricity.

Indeed, we often meet, in physical inquiries, with sources of electricity capable of affording only very feeble repulsive forces, and which fail entirely when they have attained a certain limit; but which, if we destroy the electricity thus produced, develope it anew. Of this we shall soon present several examples. Suppose a communication between one of these constant sources of electricity and the collector plate of the condenser whose insulating lamina is exceedingly thin, a single layer of varnish, for example. It is evident that the electricity from this source will go on accumulating in the condenser till the quantity not disguised is equal to what the collector plate would receive directly from the same source. Let us denote this quantity by E. When we have reached the limit in question, if we separate the condenser from the source of electricity, and remove the collector plate, its charge will be equal to the quantity E multiplied by the condensing force. It may therefore become sensible, however weak E may be, if the ratio of saturation differ little from unity, that is, if the distance between the plates of the condenser is very small, a condition which the layer of varnish perfectly fulfils.

In order to unite the indications of this instrument with those of the straw electroscope, which Volta commonly used as being the most portable and the most convenient, we unscrew the upper knob from the stem, and substitute in the place of it, the lower plate of the condenser. This plate is then insulated by the glass case of the electroscope. It is made to communicate directly by a metallic wire with the constant source of electricity, and we merely touch the upper plate to make it communicate with the ground. With this arrangement, it is the lower plate which collects the electricity. When we think the charge sufficient, we separate it from the constant source without touching it, keeping for that purpose an insulating rod; we then remove the upper plate by its insulating handle. The electricity of the lower plate, becoming free, manifests its repulsive force by E. & M.

12

Fig. 32,

the divergence of the straws. It is then easy to determine its nature by the usual tests. It is sometimes more convenient to make the constant source communicate with the upper plate of the condenser; we then touch that which communicates with the straws. When the instrument is charged, we cease to touch it; it is separated from the source of the electricity, and the upper plate is removed which carries away with it the electricity which it had acquired. Then the lower plate which is left insulated, preserves the contrary electricity and manifests it by the divergence of the straws. Its charge is, in this way, somewhat less than that of the collector plate, in the first method, since the ratio of saturation at a distance is always fractional. But the difference will not be sensible, if, as we suppose, the lamina is very thin, because this ratio will then approach exceedingly near to unity. It is only necessary to remember that this electricity is of a different nature from that of the source.

It is evident that we might equally well apply the condenser to the electroscope of Coulomb; but as the method is exactly the same, it is unnecessary to describe it here.

Of the Electrophorus.

80. When a body is electrified and insulated, if we bring toward it another body not insulated, the latter will take the contrary electricity, and if it be suddenly insulated, it will be free to be charged with this electricity. This has been shown several times in the preceding sections, and may be proved again in different ways.

We charge the conductors of the machine with a certain quantity of electricity, and bring toward them at a distance, a metallic disc supported by a glass rod. If we withdraw this disc without having touched it, it will be found to be in its natural state; but if we touch it while within the influence of the conductors, and then remove it, first taking off the hand, we shall find it charged with electricity the opposite to that of the conductors.

We take a metallic disc supported upon a stand, insulate it and give it a spark; after which we use it as in the preceding