TORPEDO, or CRAMP-FISH, has been described under the generic title RAJA; and an attempt made to explain its electrical phenomena in the article ELECTRICITY, n° 238, &c. (Both these articles are in the Encyclopædia). From some late discoveries, however, of Volta and others, the shock given by the torpedo appears much more analogous to the shock of GALVANISM than to that of common electricity; and even the electrical organs of the fish seem to resemble the apparatus with which those discoveries in galvanism were made.
In the 63d volume of the Philosophical Transactions, Mr Hunter describes the electric organ of the torpedo as consisting of a number of columns, varying in their length from an inch and a half to a quarter of an inch, with diameters about two-tenths of an inch. The number of columns in each organ of the torpedo which he presented to the Royal Society was about 470; but in a very large torpedo which he dissected, the number of columns in one organ was 1182. These columns were composed of films parallel to the base of each; and the distance between each partition of the columns was th of an inch. From these facts, the reader will find the anomalies of torpedinal electricity (supposing it the same with common electricity) accounted for in a very ingenious and philosophical manner by Mr Nicholson, at p. 358 of the first volume of his valuable Journal. We pass on, however, to point out the resemblance between it and the lately discovered phenomena in galvanism.
Take any number of plates of copper, or, which is better, of silver, and an equal number of tin, or, which is much better, of zinc, and a like number of discs, or pieces
Torpedo. pieces of card, or leather, or cloth (A), or any porous substance capable of retaining moisture. Let these last be soaked in pure water, or, which is better, salt and water, or alkaline leys. The silver or copper may be pieces of money. Build up a pile of these pieces; namely, a piece of silver, a piece of zinc, and a piece of wet card: then another piece of silver, a piece of zinc, and a piece of wet card: and so forth, in the same order (or any other order, provided the pieces succeed each other in their turn), till the whole number intended to be made use of is builded up. The instrument is then completed.
In this state it will afford a perpetual current of the galvanic influence through any conductor communicating between its upper and lower plates; and if this conductor be an animal, it will receive an electrical shock as often as the touch is made, by which the circuit is completed. Thus if one hand be applied to the lower plate, and the other to the upper, the operator will receive a shock, and that as often as he pleases to lift his finger and put it down again.
This shock resembles the weak charge of a battery of immense surface; and its intensity is so low that it cannot make its way through the dry skin. It is therefore necessary that a large surface of each hand should be well wetted, and a piece of metal be grasped in each, in order to make the touch; or else that the two extremities of the pile should communicate with separate vessels of water, in which the hands may be plunged.
The commotion is stronger the more numerous the pieces. Twenty pieces will give a shock in the arms, if the above precautions be attended to. One hundred pieces may be felt to the shoulders. The current acts on the animal system while the circuit is complete, as well as during the instant of commotion, and the action is abominably painful at any place where the skin is broken.
That this influence, whatever it may be, has a striking resemblance to the repeated shocks given by the torpedo, is obvious; but what it really is in itself must be ascertained, if it can be ascertained at all, by future experiments. Mr Nicholson indeed, from whose Journal we have taken this account of Volta's apparatus and its effects, seems confident that these effects proceed from an electrical stream or current; but this mode of operation is quite foreign from all the laws of electricity known to us. The galvanic influence in this apparatus appears to move perpetually in a circle; to which we are acquainted with no fact in electricity that is at all similar. Galvanism, too, seems capable of accumulation, even while surrounded by conducting substances, which is quite inconsistent with all that we distinctly know of electricity and its laws.
That the energy of the apparatus, however, is the effect of an electric stream or current, our ingenious author thinks proved by the condenser with which Sig. Volta ascertained the kind of the electricity, and obtained its spark. He finds the action strongest, or most pungent, on wounds on the minus side of the apparatus, or where the wounds give out electricity; a fact also observable in the common electric spark.