RETURNING stroke, in electricity, is an expression used by Lord Mahon, (now Earl Stanhope), to denote the effect produced by the return of the electric fluid into a body, from which, under certain circumstances, it has been ex pelled.
To understand properly the meaning of these terms, it must be premised, that according to the author's experiments, an insulated smooth body, - immerged within the electrical atmosphere, but beyond the striking distance of another body, charged positively, is at the same time in a state of threefold electricity. The end next to the charged body ac quires negative electricity ; the further end is positively electrified; while a cer tain part of the body, somewhere between its two extremes, is in a natural, unelec trifled, or neutral state ; so that the two contrary electricities balance each other. It may further be added, that if the body be not insulated, but have a communica tion with the earth, the whole of it will be in a negative state. Suppose, then, a brass ball, which may he called A, to be constantly placed at the striking distance of a prime conductor, so that the conduc tor, the instant when it becomes fully charged, explodes into it. Let another large or second conductor be suspended in a perfectly insulated state, further from the prime conductor than the striking distance, but within its electrical atmo sphere : let a person, standing on an insu lated stool, touch this second conductor very lightly with a finger of his right hand; while with a finger of his left hand he communicates with the earth, by touching very lightly a second brass ball, fixed at the top of a metallic stand, on the floor, which may be called B. Now, while the prime conductor is receiving its electricity, sparks pass (at least if the distance between the two conductors is not too great) from the second conduc tor to the right hand of the insulated per son; while similar and simultaneous sparks pass out from the finger of his left hand into the second metallic ball, B, commu nicating with the earth. At length, how ever, the prime conductor, having ac quired its full charge, suddenly strikes into the ball, A, of the first metallic stand, placed for that purpose at the striking distance. The explosion being made, and the prime conductor suddenly robbed of its elastic atmosphere, its pres sure or action on the second conductor, and on the insulated person, as suddenly ceases, and the latter instantly feels a smart returning stroke, though he has no direct or visible communication (except by the floor) with either of the two bo dies, and is placed at the distance of five or six feet from both of them. This re
turning stroke is evidently occasioned by the sudden re-entrance of the electric fire, naturally belonging to his body and to the second conductor, which had be fore been expelled from them by the ac tion of the charged prime entiductor upon them ; and which returns to its former place in the instant when that action or elastic pressure ceases. When the se cond conductor and the insulated person are placed in the densest part of the trical atmosphere of the prime conadc tor, or just beyond the striking distance, the effects are still more Considerable ; the returning stroke being se vere and pungent, and consi derably sharper than even the main stroke itself received directly from the prime conductor. Lord Mahon observes., that persons and animals mty he destroy, ed, and particular parts of buildings may be much damaged, by an electrical re turning stroke, occasioned even by some very distant explosion frnm a thunder cloud; possibly at the distance of a mile or more. It is certainly Mtt. difficult to conceive, that a charged extensive thun der cloud must be productive of effects similar to those produced by the prime conductor ; but perhaps the effects are not so great, nor the danger so terrible, as it seems to haye been apprehended. if the quantity of electric fluid aaturally con tained, for example, in the body of a man, were immense or indefinite then the es.
timate between the effects producible by a cloud, and those caused by a prime conductor, might be admitted; but sure ly no electrical cloud can expel from a body more than the natural quantity of electricity which this contains. On the sudden removal, therefore, of the pres sure by which this natural quantity had been expelled, in consequence of the ex plosion of the cloud into the earth, no more (at the utmost) than his whole na tural stock of electricity can re-enter his body, provided it be so situated, that the returning fire of other bodies must ne cessarily pass through his body. But per haps we have no reason to suppose that this quantity is so great, as that its sudden re-entrance into his body should destroy or injure him. See " Mabon's Electri city."