40 



ELECTRO-MAGNETISM. 



which is to act upon it. Thus, if the nee- 

 dle n s, fig. 76, be placed in the same 

 plane with the wire W w, proceeding from 

 the two cups, P, N, and bent in a circular 



Fig. 76. 



or oval form, so as to enclose it, the in- 

 fluence of every part of the wire when 

 it so surrounds the needle, will be to 

 turn both its poles in the same rotatory 

 direction, until it takes a position at 

 right angles to the plane of the figure. 

 Let this plane be directed to the mag- 

 netic north and south that is, coincide 

 with the direction which the needle 

 naturally assumes by the influence of 

 the earth when left to itself, and undis- 

 turbed by the action of any electric in- 

 fluence; and let a feeble current of 

 electricity be now sent through the wires : 

 the effect of this current will be to oc- 

 casion such a deviation of the needle 

 from the plane of the magnetic meridian 

 as will balance the force which the mag- 

 netism of the earth exerts in bringing it 

 towards that plane. In proportion as 

 the needle recedes from the meridian, 

 the terrestrial force increases in inten- 

 sity, while, at the same time, the electro- 

 magnetic force diminishes ; the number 

 of degrees at which it stops, and which 

 mark where the equilibrium between 

 these two forces takes place, will there- 

 fore indicate, with tolerable precision, 

 the intensity of the galvanic current cir- 

 culating through the wires. 



(112.) The effect of a single turn, or 

 coil of the wire may be increased by 

 multiplying the coils; for in this way 

 the same current is made to act repeat- 

 edly, in its course through the convolu- 

 tions of the wire, upon the poles of the 

 same needle. It is true that the electro- 

 magnetic force of the current is some- 

 what weakened by such an extension of 

 the line of its course ; but its diminu- 

 tion from this cause will scarcely be 

 sensible, if the total length of the wire 

 be not very considerable in comparison 

 with the whole circuit of the current 

 including the voltaic battery. In order 

 to prevent the electric current from 

 taking a shorter course than the one 

 intended, it is necessary to secure the 

 adjacent portions of the wires from 



coming in mutual contact ; for "such 

 contact would allow of the direct pas- 

 sage of the current from the one to the 

 other. For this purpose the wire must 

 either be wrapped round with silk 

 thread, or coated with sealing wax, 

 throughout the whole length of the coil. 



(11.3.) A galvanometer, constructed 

 on this principle, was invented by Pro- 

 fessor Schweigger, of Halle, very soon 

 after the first discovery of Electro- 

 Magnetism, and was called by him an 

 Electro -Magnetic Multiplier. Various 

 forms have been given to this instru- 

 ment, either with a view to increase its 

 sensibility, or to adapt it to different 

 modes of application under particular 

 circumstances. 



(114.) One of the simplest forms of 

 the instrument is that represented in 

 Jig. 77, in which a common compass- 

 needle is suspended on a pivot proceeding 

 from a wooden stand, and enclosed by a 



Fig. 77. 



great number of turns of wire, bent into 

 the shape of a vertical parallelogram, 

 and the two ends of which terminate, 

 as usual, in small metallic cups, con- 

 taining mercury, for the purpose of 

 establishing connexions with any gal- 

 vanic combination of which we are 

 desirous to ascertain and measure the 

 electrical state. A graduated circle, 

 having a dark line across it, coinciding 

 with the plane of the wires, is to be 

 fixed to the pivot, immediately under the 

 needle, in order to estimate its deviations 

 in either direction from that plane. 



(115.) Greater mobility may be given 

 to the needle by the more delicate mode 

 of suspension employed in the balance 

 of torsion. With this view, it may be 

 suspended at its centre by a fine thread, 

 or, what is best of all, by a single fila- 

 ment of silk, enclosed in a tube, and 

 attached to the lower end of a short 

 metallic wire, passed through the cover 

 which closes the top of the tube, and 

 capable of being turned in the aperture 

 with some degree of friction, so as to 

 bring the needle to any required hori- 

 zontal position. The angular turning 

 requisite for this purpose is marked by 

 an index fixed upon the upper end of the 



