158 Prof. Weber on the Application of Magnetic Induction 



Fig. 5 is a gi'ound plan of the building, showing the relative 

 situations of the inductor C, the galvanometer B, and the tele- 

 scope and scale A. The distance between the galvanometer 

 and inductor is so great, that the influence of the galvanometer 

 needle on the indiictor vanishes in comparison to that of the 

 earth's magnetism. The inductor stands on three foot-screws 

 a, b, c, which are used to give the axis of the inductor a perfectly 

 vertical or horizontal jiosition, according as the horizontal or rer- 

 ^zcff/ component of the earth's magnetic force is used for induction. 



It was so contrived, that during the obseiTations the person 

 at the telescope could set the inductor in rotation by means of 

 his foot ; in the following obser\'ations, however, the induction 

 strokes were made by an assistant. The two wires from the 

 galvanometer and inductor passed over the telescope-stand close 

 to the observer, where, by means of a clamp, the galvanometer 

 wire could be made to return without passing over the inductor, 

 or a continuous wire made to pass over both the galvanometer 

 and inductor. As these contrivances, however, can be modified 

 to suit the conveniences of the observatory and habits of the 

 observer, a fuller description is unnecessary. 



It should be here remarked, that, for inclination measure- 

 ments alone, a galvanometer with so great a helix as the one 

 above described is by no means necessary ; it is only because 

 the mean radius of the coils ought not to be less than 230 

 millimetres that so great a mass of copper (60 kilogrammes) 

 was used for this galvanometer. But the great width of the 

 coil, as well as its circular form, was not chosen on account 

 of the inchnation measurements to be made therewith, but 

 because it was intended that the instrument should also serve 

 for fixed standard measurements of the normal resistance in gal- 

 vanic circuits, similar to the instrument described in the Ab- 

 handlunyen iiber elektrodijnamische Mausbestimmungen (Leipzig, 

 1852), ii., with which, however, such measurements of resistance 

 could be made but once. Such measurements have actually been 

 made with the present instniment ; but for the results, as well 

 as for the necessary theoretical elucidations, we must refer the 

 reader to the original memoir. 



For correct and convenient measurements of the inclination, 

 which at present interests us, Weber remarks that all the dimen- 

 sions of the galvanometer may without injury be diminished 

 one-half; so that an eighth part of the wire (about 10 kilo- 

 grammes) would be sufficient. The accuracy of the inclination 

 measurements would even be increased by this means, for the de- 

 flection of the needle Avould be thereby increased in the ratio of 

 1 : V2, and might perhaps be doubled, if, instead of the cir- 

 cular form, the coil had an appropriate elliptical form, the 



