42 HENEY A. ROWLAND 



applied according to the formula of Blanchet and De la Prevostaye for 

 the length of the needle, the position of the poles being estimated; this 

 correction in the deflections used was always less than -6 per cent. To 

 eliminate any error in the position of the zero-point, two readings were 

 always taken with the currents in opposite directions, each one being 

 estimated with considerable accuracy to ^ of a degree. 



The experiments were carried on in the assay laboratory of the 

 Institute, which was not being used at that time; and precautions were 

 taken that the different parts of the apparatus should not interfere 

 with each other. The disposition of the apparatus is represented in 

 Plate II. 



The current from the battery A, of from two to six large Chester's 

 " electropoion " cells No. 2, joined according to circumstances, passed 

 to the commutator B, thence to the tangent-galvanometer C, thence 

 to another commutator D, thence around the magnet E (in this case a 

 ring), and then back through the resistance-coils K to the battery. To 

 measure the magnetism excited in E, a small coil of wire F was placed 

 around it, 11 which connected with the galvanometer H, so that, when 

 the magnetism was reversed by the commutator D, the current induced 

 in the coil F, due to twice cutting the lines of force of the ring, 

 produced a sudden swing of the needle of H. As the needle swung 

 very freely and would not of itself come to rest in ten or fifteen min- 

 utes, the little apparatus 7 was added : this consisted of a small horse- 

 shoe magnet, on one branch of which was a coil of wire ; and by sliding 

 this back and forth, induced currents could be sent through the wire, 

 which, when properly timed, soon brought the needle to rest. This 

 arrangement was very efficient; and without it this form of galvano- 

 meter could hardly have been used. To compare the magnetism of 

 the ring with the known magnetism of the earth, and thus reduce it to 

 absolute measure, a ring G supported upon a horizontal surface was 

 included in the circuit; when this was suddenly turned over, it produced 

 an induced current, due to twice cutting the lines of magnetic force 

 which pass through the ring from the earth's magnetism. The induced 

 current in the case of either coil, F or G, is proportional to the number 

 of the lines of force cut by the coils " and to the number of wires in the 

 coil, which latter is self evident, but may be deduced from the law of 

 Gaugain. 1 * It is evident, then, that if c is the deflection from coil G, 



11 If a bar was used, this coil was placed at its centre. 



12 Faraday's Experimental Researches, vol. iii, series 29. 

 13 Dagnin's Traite de Physique, vol. iii, p. 691. 



