540 LECTURE LV. 



action which we have de- 

 scribed, it is evident that the 

 effect can he multiplied al- 

 most indefinitely hy simply 

 coiling the wire and placing 

 the needle within the coil ; 

 for the currents on each side 

 of the needle all tend to move 

 it in the same direction. 



In this way the galvano- 

 meter is constructed.* A 

 small needle is suspended hy 

 a fibre of silk, and a coil of 

 wire, coated with sealing wax 

 or silk, causes the voltaic current to circulate in directions parallel to it. 

 The tangential action of the current overcomes the magnetic action of the 

 earth and deflects the needle. The delicacy and value of this instrument 

 have been greatly increased by the inventions of Gumming t and Nobili.J 

 Instead of a single needle, two needles are used, which are placed with 

 their poles opposite ways, so that the directive tendency due to the earth's 

 action is completely neutralized, and the torsion of the suspending thread 

 is the sole impediment to motion. In the figure the needles are ordi- 

 nary sewing needles similarly magnetized, and passed parallel to each 

 other through a flat bit of straw which is attached to the fibre of silk. The 

 coil of wire passes about the lower needle, having in its upper part an 

 opening through which the straw passes freely. The amount of force 

 exerted is the sum of the actions of the upper and lower currents on the 

 lower needle, together with the difference of those on the upper wire. 

 The action of the voltaic current on a magnetic needle is very similar to 

 the action of one magnetic needle on another, except that it is perpendi- 

 cular to the direction of the current. Now the action of a magnetic needle 

 produces the magnetic state in a bar of soft iron, and hence it is natural to 

 conclude that a voltaic current should produce a similar state. Accord- 

 ingly, if a considerable quantity of copper wire be twisted round a piece of 

 soft iron bent into the form of a horseshoe, and a voltaic current be passed 

 along the wire, the result is the formation of a powerful magnet. On 

 discontinuing the communication with the voltaic pile, the iron is instantly 

 reduced to nearly its former state. This presents us with a promising field 

 of research in its applicability to economical purposes as a moving power. 

 And although the endeavours of Jacobi and others have as yet been only 

 partially successful, there is every reason to suppose that time will 



* Schweigger, in his Jour. 1821. f Camb. Tr. 1821, p. 281. 



J Memorie ed Osservazioni colla Descrizione de suoi Apparati, 2 vols. Firenze, 

 1834. See also Melloni, Arch, de 1'Electr. i. 165. 



Ritchie, Phil. Mag. iv. 13. Dal Negro, Nuova Macchina Elettro-Mag. Ann. 

 delle Scienze del Regno Lomb. Venet. 1834. Jacobi, Mem. sur 1' Application de 

 1'Electro-Mag. au Mouvement des Mach. Potsdam, 1835. Sturgeon, in Stuf.-Ann. 

 i. 75 ; viii. 81. Davenport, ibid, ii. 284. Davidson, Mechanics' Magazine, Nov. 

 1842. 



