66 WRITINGS OF JOSEPH HENRY. [1831 



The whole will be more readily understood by a reference 

 to the annexed drawing: a 6 is the horizontal magnet, 

 about seven inches long, and movable on an axis at the 

 centre : its two extremities when placed in a horizontal line, 

 are about one inch from the north poles of the upright 

 magnets c and d. g and / are two large tumblers contain- 

 ing diluted acid, in each of which is immersed a plate of 

 zinc surrounded with copper. I, m, s, t, are four brass thim- 

 bles soldered to the zinc and copper of the batteries and filled 

 with mercury. 



The galvanic magnet ab is wound with three strands of 

 copper bell-wire, each about twenty-five feet long ; the simi- 

 lar ends of these are twisted together so as to form two stiff 

 wires, which project beyond the extremity b, and dip into 

 the thimbles s, t. 



To the wires q, r, two other wires are soldered so as to pro- 

 ject in an opposite direction, and dip into the thimbles I, m. 

 The wires of the galvanic magnet have thus, as it were, four 

 projecting ends ; and by inspecting the figure it will be seen 

 that the extremity p, which dips into the cup m attached to 

 the copper of the battery in g corresponds to the extremity 

 r connecting with the zinc in /. 



When the batteries are in action, if the end b is depressed 

 until q, r dips into the cups s, t, ab instantly becomes a 

 powerful magnet, having its north pole at b; this of course 

 is repelled by the north pole d, while at the same time it is 

 attracted by c, the position is consequently changed, and 

 0, p comes in contact with the mercury in I, m; as soon as 

 the communication is formed, the poles are reversed, and 

 the position again changed. If the tumblers be filled with 

 strong diluted acid, the motion is at first very rapid and 

 powerful, but it soon almost entirely ceases. By partially 

 filling the tumblers with weak acid, and occasionally adding 

 a small quantity of fresh acid, a uniform motion, at the rate 

 of seventy-five vibrations in a minute, has been kept up for 

 more than an hour: with a large battery and very weak 

 acid, the motion might be continued for an indefinite length 

 of time. 



