492 MEMORIAL OF JOSEPH HENRY. 



was withdrawn from the acid, and again deflected to the west when 

 it was re-immersed. This operation was repeated many times in suc- 

 cession, and uniformly with the same result, the armature the whole 

 time remaining immovably attached to the poles of the magnet, no 

 motion being required to produce the effect, as it appeared to take 

 place only in consequence of the instantaneous development of the 

 magnetic action in one and the sudden cessation of it in the other. 

 - - - From the foregoing facts it appears that a current of 

 electricity is produced for an instant in a helix of copper wire sur- 

 rounding a piece of soft iron whenever magnetism is induced in the 

 iron; and a current in an opposite direction when the magnetic 

 action ceases ; also that an instantaneous current in one or the other 

 direction accompanies every change in the magnetic intensity of the 

 iron." 



I will now give Henry's iaccount of the experiment by which he 

 obtained a spark from the magneto-electric current — certainly the 

 first flash of a magneto-electric current ever seen in this country: 

 " The poles of the magnet," says Henry, " were connected by a single 

 rod of iron bent into the form of a horse-shoe, and its extremities 

 filed perfectly flat so as to come in perfect contact with the faces of 

 the poles: around the middle of the arch of this horse-shoe two 

 strands of copper wire were tightly coiled, one over the other. A 

 current from one of these helices deflected the needle one hundred 

 degrees, and when both were used, the needle was deflected with such 

 force as to make a complete circuit. But the most surprising effect 

 was produced when instead of passing the current through the long 

 wires to the galvanometer, the opposite ends of the helices were held 

 nearly in contact with each other and the magnet suddenly excited : 

 in this case a small but vivid spark was seen to pass between the 

 ends of the wires, and this effect was repeated as often as the state 

 of intensity of the magnet was changed. - - - It appears 

 from the May number of the Annals of Philosophy, that I have 

 been anticipated in this experiment of drawing sparks from the 

 magnet by Mr. James D. Forbes, of Edinburgh, who obtained a 

 spark on the 30th of March, my experiments being made during the 

 last two weeks of June. A simple notification of his result is given, 

 without any account of the experiment, which is reserved for a 



