Scientific Lectures. 147 



spools, each wrapped with nine layers of stout copper wire. Thus 

 we have eight spools, containing in all 2,000 feet of 2-10 inch insu- 

 lated wire. Four of these hollow spools are placed in a line on each 

 table, and into them is introduced a hollow cylinder of very soft iron 

 three feet three inches long, of six inch outside diameter, and with an 

 interior diameter of three and a half inches.* The two cylinders are 

 terminated with these conical caps, and by rolling the tables they can 

 be placed at various distances apart. On the shelves under these 

 tables are twelve large battery-cells containing a solution of bi-chromate 

 of potassa in dilute sulphuric acid. By turning these handles, those 

 large plates (ten pairs to each cell) of zinc and carbon can be lowered 

 into the solution and a powerful development of electricity instantly 

 follows, and by means of these wire ropes it is led in one direction 

 through all the wire on the eight spools. The electricity thus flows 

 (as we say) around the two iron cores, and whenever this takes place 

 they are instantly endowed with magnetic properties of the greatest 

 intensity, as you will soon see. 



The plates are out of the solution, and I place against the core 

 these iron spikes. On ceasing to support them with my hands they 

 fall to the ground, for the iron cores have no power to hold them up. 

 Now I lower the plates into the solution, the electricity courses 

 through the spools, and see, how the spikes are jerked out of my 

 hand as I bring them near the magnet, and it takes all my strength 

 to detach even one of them. I now throw a score of these spikes at 

 the cores, which you will observe are about eighteen inches apart. 

 How they clash against the iron ! My assistants will now lift a whole 

 keg of these eight-inch spikes above the maget, and turning it upside 

 down they rush to the iron cores, and not one of them has fallen to 

 the ground. Another keg of spikes is now thrown on the magnet, 



* This most remarkable magnetizing power of an electric current was discovered by Arago in 

 1820, who found that as long as the current encircled an iron wire the latter remained magnetized, 

 and that in similar circumstances a needle of steel received a permanent magnetic charge. The 

 full development, however, of this discover}' of Arago we owe to the genius of Professor Joseph 

 Henry, who subsequently, in 1831, discovered the conditions necessary to obtain the greatest mag- 

 netic effect from any given bar of iron with any given battery; and, guided by these discoveries, he 

 constructed a magnet which supported nearly three tons. Thus to our countryman belongs the 

 honor of being the first to present freely to the world the knowledge of these fundamental facts, 

 absolutely essential for the subsequent invention of the electro-magnetic telegraph ; which inven- 

 tion, in all its essential principles, is also due to Henry, who, in 1839, at Albany, and during the 

 following years at Princeton, exhibited his apparatus for transmitting electro-magnetic signals to a 

 distance. Subsequently, Morse, backing his mechanical genius by an undaunted perseverance, suc- 

 ceeded in embodying the discoveries of Henry in his efficient invention, which has given to Ameri- 

 can ingenuity such world-wide renown. The powerful magnet which I used in this lecture is 

 essentially Henry's magnet of 1832. It differs from it only in having had removed from its iron 

 cores their inactive central portions, and in the trivial difference of these cores being placed, merely 

 for convenience, in a horizontal position. See Rept. of Regents, Smith. Inst., 1857, p. 85, el seq. 



