CHAP, in.] ELECTRICITY AND MAGNETISM. 



169 



meters devised by Sir. W. Thomson for signalling through 

 submarine cables, are admirable examples of this class 

 of instrument. In Fig. 91 the general arrangements of 

 this instrument are shown. The body of the galvano- 

 meter is supported on three screw feet by which it can 

 be adjusted. The magnet consists of one or more 

 small pieces of steel watch-spring attached to the back 



of a light concave silvered glass mirror about as large 

 as a threepenny piece. This mirror is hung by a single 

 fibre of cocoon silk within the coil, and a curved magnet, 

 which serves to counteract the magnetism of the earth, 

 or to direct the needle, is carried upon a vertical support 

 above. Opposite the galvanometer is placed the scale. 

 A beam of light from a paraffin lamp passes through 

 a narrow aperture under the scale and falls on the 

 mirror, which reflects it back on to the scale. The 

 mirror is slightly concave, and gives a well defined spot 

 of light if the scale is adjusted to suit the focus of the 



