492 WEBER ON THE MEASUREMENT 



many times around the coil, and thus a very strong multiplica- 

 tion of the galvanic force be obtained. Moreover, this rotating 

 coil may without injury be loaded with a speculum, which also 

 rotates, and here, as in Gauss's magnetometer, may be used for 

 the delicate measurement of angles ; for provided friction be 

 excluded, the application of delicate optical instruments in this 

 case also does not form any impediment. Regarding the con- 

 struction of the instrument in detail, as this has been described 

 very perfectly by M. Leyser, the instrument-maker in Leipzic, 

 I shall insert the explanation which he has given, and which 

 refers- to the figures sketched by him, Plate III. fig. 1-10. The 

 instrument is called an Electro-dynamometer. 



Description of the Electro-dynamometer. 



Fig. 1, Plate III., represents the little frame for supporting 

 the reel which vibrates in the multiplier, seen diagonally. This 

 frame consists of two round ivory discs, a a and a a, which are 

 riveted to two ivory plates, b V and h b' ; their distance apart is 

 regulated by a small ivory roller, c. The latter is hollow, so that 

 a metallic I'od can be passed through it, and by means of a screw 

 each of the discs with its plate can be fixed to the ends of the 

 roller ; and thus a reel is formed for the reception of the wire. 

 One end of the wire to be coiled passes through the small hole d, 

 and projects from it. When the wire is placed upon the reel 

 and the end fixed by means of silk, the metallic supports, eee 

 and e e e', of the reel are fixed to the ends of the plates above 

 mentioned; thus, one support, eee', to which the speculum// 

 is screwed at g, is riveted at b' V ; whilst the other support, eee, 

 to which the counterpoise /* h is fixed by the screw i, is fastened 

 by screws at b b; so that this support, near the screws b b, may 

 be thrown back in the direction b b', in order that the entire reel 

 may be conveniently placed in the multiplier. The commencement 

 of the reel, which was left projecting through the hole at d, is 

 now placed lengthwise along a portion of the plate b V towards V, 

 until the circumference of the reel admits at k of its being again 

 placed within the frame and then ascending to the support of 

 the speculum, where bj'^ means of a small screw nil above the 

 point at which the speculum is fixed, it comes into metallic con- 

 tact with the support. The end of the reel is also brought into 

 metallic contact with the other support by means of the screw m ; 

 this end must however be long enough not to stand in the way 



