450 Mr. J. P. Joule's Account of Escperiments 



means of screws, weights, or by the attraction or repulsion of a 

 steel magnet kept for the purpose. Then on making the ne- 

 cessaiy battery communications at cc, the scale s will rise with a 

 force estimated by the weight, in grains, which is required to 

 bring the magnet again to zero. In my particular instrument, 

 I have found that one degree of current is indicated by 0*69 of 

 a grain. 



The value of this new galvanometer (the sensibility of which 

 may be increased at pleasure by multiplying the number of coils), 

 besides its usefulness in measuring copious currents, consists 

 chiefly in its perfect independence of the terrestrial magnetism, 

 as well as of any magnetic influence of surrounding bodies. In 

 every possible situation, provided that the intensity of the ba- 

 lance bar is constant, and that no interference is induced after 

 the adjustment to zero, the transmitted current will be exactly 

 proportional to the weight lifted by the scale ; and I should have 

 as much confidence in working with it on an iron steam-boat as 

 if eveiy particle of iron were entirely removed from it. 



I now proceed to describe my electro-magnets, which I had 

 occasion to construct of very different sizes, in order to develope 

 any curious circumstance v/hich might present itself. A piece 

 of cylindrical wrought iron, 8 inches long, had a hole 1 inch 

 in diameter bored through the entire length of its axis; one 

 side of it was then planed away, until the hole was laid open 

 through its entire length. Another piece of iron, also 8 inches 

 long, was then planed ; and having been secured with its face in 

 contact with the other planed surface, the whole was turned into 

 a cylinder 8 inches long, 3f inches in exterior diameter, and \\ 

 inch in the diameter of the bore. The larger piece (which was 

 intended for the electro-magnet) was then wound with four 

 copper wires, each of which was 23 feet long and yV^h of an inch 

 in diameter, and co- 

 vered with silk. This Q 

 electro-magnet*, which 

 I shall designate No. 1, 

 is along with its arma- 

 ture represented by 

 the accompanying fi- 

 gures. 



I constructed an- 

 other electro-magnet 

 (No. 2) of a piece of round iron bar, half an inch in diameter 

 and 2*7 inches long. It was bent into an almost semicircular 

 shape, and covered with 7 feet of well-insulated copper wire J^th 

 of an inch thick. The poles were half an inch asunder. 



* This electro-magnet is at present on view in the Exhibition of Industry 

 of all Nations.— May 1861, J. P. J. 



