ELECTRO-MAGNETISM. 



is, that both needles are acted upon in 

 the same manner, as far as the rotatory 

 tendency is concerned, by the adjoining 

 wires. The lower needle, being in the 

 Situation similar to that in the simple 

 apparatus already described, 91, is 

 acted upon by the sum of the forces of 

 the currents in every part of the coil. 

 The upper needle, placed, with regard to 

 the wires, in the same situation as in 

 Mr. Ritchie's galvanometer, is acted 

 upon by the excess of force in the upper 

 current which is nearest to it. This 

 force acts upon it in a direction the 

 reverse of that in which it acts upon 

 the lower needle, because it is situated 

 on the opposite side ; but since the poles 

 are also in a reversed position, the rota- 

 tory action becomes the same in its 

 direction on both needles. Hence, be- 

 sides the increase of sensibility in con- 

 sequence of the removal of the greater 

 part of the opposing force derived from 

 the magnetic influence of the earth, we 

 have also an increase of power from the 

 addition of the upper needle. There is 

 also a convenience in employing the 

 upper needle as an index ; for by allow- 

 ing of the graduated circle being placed 

 above, instead of within the frame, the 

 folds of the wire may be brought much 

 nearer to each other than in the com- 

 mon instrument : this renders it more 

 compact, and, from the greater approxi- 

 mation of the lower needles to the wires, 

 also more powerful. The estimation of 

 the deflection of the needle by reference 

 to the graduated circle, can also be more 

 conveniently made, from the view not 

 being obstructed by the presence of the 

 wires above the needle, as in the ordi- 

 nary construction. It is hardly neces- 

 sary to observe, that when fixing the 

 graduation, the zero point should be 

 placed so as to accord with the position 

 of the upper needle, when left to the 

 undisturbed action of the magnetism of 

 the earth. 



(121.) It is evident that if the mag- 

 netic powers of the two needles employed 

 in Nobili's galvanometer were perfectly 

 equal, they would exactly neutralize each 

 other, as far as regards the directive 

 influence of the earth ; and the system 

 of needles would be indifferent to any 

 position. This would, however, defeat 

 the purpose of the instrument, the object 

 of which is to measure a feeble electro- 

 magnetic force, by putting it in equili- 

 brium with another force, likewise feeble, 

 but still acting, and susceptible of mea- 



surement. If, 'therefore, a perfectly as- 

 tatic needle, that is, one which retains 

 no directive power whatever, be em- 

 ployed, it will be necessary to find some 

 other weak, but variable and easily-mea- 

 surable force to obtain this equilibrium. 

 Such a force is that of torsion ; and 

 accordingly the greatest degree of per- 

 fection attainable in the measurement 

 of minute electro-magnetic forces, would 

 appear to be obtained by applying to the 

 apparatus of Nobili, the needles being 

 previously rendered perfectly astatic, 

 the principle of the torsion suspension 

 adopted by Mr. Ritchie. 



(122.) An extension of the principle 

 of Nobili's galvanometer has been pro- 

 posed by M. Lebaillif ; who employs a 

 combination of four needles instead of 

 two ; one pair being applied to the upper 

 part of the coil, and one pair to the lower, 

 in the manner exhibited in the section, y?g\ 

 81, where N S, SN, represent the upper 

 Fig. 81. 



pair of the magnets, having their poles 

 in opposite directions ; and S N, N S, 

 the lower pair, likewise reversed in their 

 polarities ; the two intermediate mag- 

 nets, which are within the coils of the 

 wire, having their poles similarly situ- 

 ated, as is likewise the case with the 

 uppermost, and lowermost magnets ; 

 and the whole being affixed to the same 

 vertical axis, which is a piece of straw, 

 passing freely through the wires, and 

 through the graduated circle, which 

 forms the top of the frame enclosing 

 both wires and needles. An index is 

 fixed in the upper end of the axis to 

 point out the positions of the needles. 

 The axis itself is suspended from the 

 end of a horizontal arm, proceeding from 

 an upright pillar at the side of the ap- 

 paratus. In order to form the coil, M. 



