and on a peculiar Condition of Magneto-electric Induction, 351 



the magneto-electric shock. This effect I have felt produced 

 by Mr. William Jenkins in a manner that was new to me ; and 

 as he does not intend to work out the result any further, but 

 has given me leave, through Mr. Newman, to make it known 

 to you, I think the sooner it is published the better. Mr. 

 Jenkins's apparatus consists of a helix cylinder formed of cop- 

 per wire in the usual manner. An iron rod, about 2 feet long 

 and half an inch in diameter, can be passed at pleasure into the 

 centre of this cylinder. The helix consists of three lengths of 

 wire, (which, however, might as well be replaced by one thick 

 wire,) the similar ends of which are soldered to two thicker 

 terminal wires, and on these are soldered also two short cop- 

 per cylinders, to be held in the hand and give extensive con- 

 tact. The electro-motor was a single pair of plates, exposing, 

 perhaps, 3 square feet of surface on both sides of the zinc 

 plate. On holding the two copper handles tightly in the 

 hands, previously moistened with brine, and then alternately 

 making and breaking the contact of the ends of the helix with 

 the electro-motor, there was a considerable electric shock felt in 

 the latter case, i. e. on breaking contact, provided the iron 

 rod were in the helix ; but none either on making or breaking 

 contact when the latter was away. 



This effect appears very singular at first, in consequence of 

 its seeming to be the shock of the electricity of a single pair 

 of plates. But in reality it is not so. The shock is not due 

 to the electricity set in motion by the plates, but to a current 

 in the reverse direction, induced by the soft iron electro- 

 magnet at the moment when, from the cessation of the original 

 current, it loses its power. It is, however, very interesting 

 thus to observe an original current of electricity, having a very 

 low intensity, producing ultimately a counter current having 

 an intensity probably a hundredfold greater than its own, and 

 the experiment constitutes one of the very few modes we have 

 at command of converting quantity into intensity as respects 

 electricity in currents. 



It has been generally supposed that the electric spark pro- 

 ducible by a single pair of plates can only be obtained upon 

 breaking contact; but this, as I have shown in the Eighth Series 

 of my Experimental Researches, is an error, and a very im- 

 portant one as regards the theory of voltaic electricity ; it is, 

 however, true that the spark upon breaking contact can be 

 very greatly exalted by circumstances having no such effect 

 upon that produced at the moment of making contact. 



Every experimenter on electro-magnetism is aware, that 

 when the current from a single pair of plates is passed through 

 a helix surrounding a piece of soft iron (to produce an electro- 



