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brackets; and when the moisture is evaporated, the rods are taken up there with the insulating hooks, and thus applied to the column.
These experiments are particularly useful for a better knowledge of insulation, a point very important in the construction of electric apparatusses; for many experiments fail for want of a complete insulation ; and I do not know of any shorter and surer method of trying the isulating faculty of varnishes laid on glass for this purpose, than that of applying to the column rods of glass covered with them. I shall also give only a general idea of this class of trials under the following head.
Exp. 12. It is very seldom, that a naked glass rod, being placed on the hooks of the column, does not sensibly diminish, in a little time, the divergence in the electroscopes, by transmitting slowly some electric fluid from A to B : but this is more or less, according to the nature of the glass; therefore these differences may become a particular object of experiments concerning the property of different glasses for electric purposes, by comparing the effects of different rods on the electroscopes of the column ; much care being taken, that all effect of friction be dissipated. With respect to varnishes, a proper sealing wax laid over the glass when hot is the best coating which I have found ; a rod of this kind produces no change in the electroscopes. But all sealing waxes have not the same property, and before I had devised this mode of trial, I was sometimes disappointed in the construction of the small set of electric apparatusses, which I have mentioned in the former paper, even in that of the electroscopes of the column, as the top of their glass bottle must be insulating. The sealing wax reckoned the finest, because it melts more easily and spreads more smoothly, is not fit for this purpose, its softness being produced by spirits of wine. In general, for this essential choice of an insulating coating on glass, the column is very useful ; for by laying the different coatings on glass rods, and placing these on the hooks of the column, those which will be found to diminish the divergence in the electroscopes are not completely insulating, and that coating must be used, by which the divergences are not affected.
I come to the impression produced on the column itself, when there remains some effect of friction on the insulating rods applied to it. The experiments on this object will, at the same time, afford a new verification of the cause which I have assigned to the motion of the electric fluid in the column (or the pile), by what is called the electric influence ; the laws of which, first really determined by Sig. Volta, I have explained by some modifications of the vector, which will be perceived in the following experiments ; showing at the same time the effect of the ambient air, such as I have determined it. The proper time for these experiments is also when, from the electric state of the ambient air, there is not much divergence in the electroscopes of the column. They may be made with naked glass, which friction renders positive ; and with a glass rod covered with sealing wax, which thus becomes negative. I shall explain the phenomena produced by the latter, because it retains longer the effect of friction ; those produced by naked glass are of the same nature, only reversed.
Exp. 13. A glass rod covered with sealing wax must be rubbed so gently, that, when applied to the column, it only increases the divergence of the gold leaves at the negative side, without any striking, else the effects would be confused : when it produces the proper effect, the phenomena are the following.
1. At the moment when the rod is placed, the divergence increases at the negative side, and diminishes, or even ceases at the positive side. The cause of these effects is, that the expansive power of the electric fluid is lessened in the whole column, by part of its vector passing to the negative rod, where its quantity is less : and as however the equilibrium of the electric fluid in the column requires more electric matter on zinc than on copper, the latter, at the first moment, loses more of it, but not sufficiently to compensate the loss of expansive power at the zinc side ; therefore less electric fluid passes into the gold leaves of the latter, which fall in consequence of this diminution.
2. Within a little time, the positive divergence is renewed at the zinc side, and the negative lessened at the copper side. This effect is produced by the ambient air, which during the diminution of expansive power in the column, yields to it some electric fluid, especially to the copper side which was become strongly negative ; and thus the former equilibrium is restored in the column.
The proof of the above explanations of the phenomena observed, which embrace the whole system, both of the motion of the electric fluid in the column, and of the influence of the ambient air on this instrument, is obtained by suddenly moving the rod : for the new quantity of electric fluid communicated to the column by the air during the influence of the negative body makes both gold leaves strike at once on the zinc side by a very great divergence.
The last object concerning the theory of the electric effects in both the column and the pile, which remains to be considered, relates to the difference between the effects of the number of groups and the size of the plates, considered under two distinct points of view, on one of which I have already given an experiment in my former paper: but I have repeated it in a different manner, which will confirm my system on these phenomena.
Exp. 14. The column of 600 groups, represented in the figure, has been composed of three columns of 200 groups each, which I had used separately from the beginning of these experiments ; but before they were united together for the purpose of the aerial electroscope, I tried their effects in the three following combinations.
1. I applied successively to the same electroscope the same extremity (either positive or negative,) of each column, the opposite extremity being placed in communication with the ground ; and I observed the quantity of divergence produced by each column, which was nearly the same.
2. I applied the three columns at the same time to the electroscope, each of them remaining in communication with the ground; and the divergence was not greater than it had been with the most active of the single columns.
3. But having connected the three columns as one, by placing conductors between their opposite extremities, and connecting one extremity of the whole with the electroscope, the other being in communication with the ground; the effect was so much increased that the gold leaves struck the sides of the electroscope.
This proves, under a different form, the same proposition which I had stated in my former paper, namely, that the simple divergence in electroscopes depends only on the density of the electric fluid, and the density on the number of groups; at the same time that it confirms the cause which I had assigned to these effects; and as they are analogous to many kinds of phenomena, I shall use another example to explain it, that of pumps. As the height to which water can be raised by pumps does not depend on either their number or size, but on their length; so in the above experiment, with three concurrent columns of 200 groups, the density of the electric fluid was not increased on one extremity, nor consequently the divergence at either extremity, more than with one column; nor could more have been done with one column of the same number of groups of whatever size. But as, in taking the water at the same level, a pump of thirty feet will bring it three times as high as three pumps of ten feet; so in the above experiment, the column of 600 groups produced probably three times as much change of density in the electric fluid, with a proportional divergence, as did the three columns of 200 groups, individually acting on the same low level, or degree of density, that of the standard of plus or minus.
But if in this case the size of the plates, or the multiplication of their number at the same numerical distances from the extremities, be indifferent, it is not the same in some other cases, as I shall illustrate by the same analogy. In the above case, the height to which the water was to be raised being the only object, the number or size of the pumps was indifferent; but if a current is to be produced at that level, either with a certain degree of rapidity, or of a certain volume, then the diameter of the pumps comes in as a condition. The following experiment will show the analogy of this case, with the effects of the different sizes of plates in the column.
Exp. 15. I made two other columns of 200 groups each ; but these I only cut square, for one of \, and for the other of į of an inch, still zinc and Dutch gilt paper. These two columns produced sensibly the same divergence as the former, in the same electroscope; but in this was already shown the difference in other respects; the time for producing this divergence was in the inverse ratio of the size of the plates.
This experiment gives a clear idea of the effect produced by a greater size of the plates, both in the pile and in the column. In the circuit of the former, with the same number of groups, the effects are proportional to the size of the plates, because the current of the electric fluid becoming denser and more rapid in passing through the wires used in these operations, the effects are greater, in proportion to the number of equal parts of surface, either in a few or many plates, concurring to produce the motion of the electric fluid which arrives at the entrance of this narrow channel. That difference in the rapidity and density of the current cannot be discovered in the circuit of the column, because the condition to which chemical effects are owing is wanting in it, as I have explained in my former paper ; but the size of the plates influences the frequency of the strikings of the little electroscopic pendula, when their simple divergence is exceeded; because each time that one of them strikes, either at the negative or the positive side, that instantaneous communication of the column with the ground changes in some degree its electric state ; and the same state is sooner restored, to produce another striking, in proportion to the size of the plates, with the number of groups. This effect will enter as an essential circumstance into the second part of this paper, concerning the aerial electroscope.
On the Ignition of Gunpowder by the Electrical Discharge ; and on
the Transmission of Electricity through Water, foc. By WILLIAM
STURGEON.* (Experimental and Theoretical Researches in Electricity, Magnetism, &c.,
continued from page 346, vol. viii.] It is generally admitted that the present state of knowledge relative to the phenomena of electricity is enveloped in much obscurity ; and perhaps no instance of electrical action manifests our ignorance of this branch of science more than that of igniting gunpowder. Yet so little notice is taken of this isolated fact, that no satisfactory attempt at explanation, that I am aware of, has ever appeared in the pages of any writer on this subject.
That gunpowder has frequently, by various individuals, been ignited by the electric fluid, is a truth that cannot be denied. But why those experimenters happened to succeed, and why others so frequently and still more constantly fail, are circumstances the cause of which has hitherto been left unexplained, -perhaps not understood. I am well convinced that no individual experimenter has been more embarrassed than myself, by fruitless attempts to ignite gunpowder by the electric fluid : and although I have varied the experiment according to all the directions I could either read or hear of, yet I candidly confess that I never succeeded by any of them.
Fruitless as these experiments were with respect to the object in view, it was observed, by passing the discharge of a jar through water (which is the method given by some authors), that the force of the shock is considerably abated; and that the report is very trifling when compared to that which is heard by a similar discharge
• From the Phil. Mag.
through metal. Convinced by this circumstance that the nature of the discharge is modified in a peculiar manner by passing the fluid through water ; yet, as I had never succeeded in igniting gunpowder by a discharge through the aqueous medium, it appeared evident that something more than this fact was necessary to be understood. I therefore became desirous to ascertain, if possible, the real cause why other experimenters succeeded, and why every attempt that I had made proved unsucceessful.
It is well known, that if a red hot iron be applied to gunpowder, the latter does not immediately ignite, but that some interval of time (however small), does certainly elapse before one single grain is on fire : and that it is possible for a red hot iron to be passed over the hand with such velocity as to produce scarcely any sensation of heat. Hence my first object, now, was to devise some means of retarding the velocity of the electric fluid : for I considered that if this could be acomplished, more time would be afforded for the fluid and gunpowder to be in contact, and the latter, in consequence, more likely to be ignited. I likewise supposed that if the electric fluid be retarded by being transmitted through water, it was likely that a jar would not be quite discharged by a very sudden contact; as it was probable that if the discharging rod was quickly withdrawn from the knob of the jar, that the whole of the fluid would not have time to make its escape. But several trials in this way, through a large tub of water, seemed to discharge the jar as effectually as if the whole circuit had been of metal.
Although these experiments were by no means satisfactory, yet I always observed that the report was much feebler, and that gunpowder placed in the circuit was not blown or scattered to so great a distance when the discharge was made through water, as when a similar discharge was transmitted through metal. Hence it was obvious that the force had by some means abated ; but whether by retardation, or by some change in the physical character of the electric fluid, I was at that time unable to determine.
Some time afterwards an idea suggested, that if the electric fluid be retarded at all by passing through water, perhaps the water possesses this property in consequence of its inferior conducting capacity with respect to metals and other good conductors ; and if so the velocity of the electric fluid might be reduced to almost any degree, by reducing the diameter of the column of water through which it had to pass. For it is evident that the conducting power of any body will be proportional to its natural capacity, and to the quantity employed at any one point in the circuit. For a discharge that will destroy a thin wire, would be conducted with safety by a wire of the same kind of metal, of greater dimensions. It now occurred that those persons who had ignited gunpowder by the electric fluid, perhaps succeeded by using very narrow tubes filled with water. (For I had frequently transmitted a discharge through a wide tube without success; and as no author gives any dimensions of the water