TELEGRAPHIC APPARATUS. 
215 
reversed; the copper of the battery is now put to earth, and 
the zinc connected with the line; consequently the needle at 
the distant station is deflected in the opposite direction. 
Fig. 152 represents a key on the same plan, which is used 
with Sir W. Thomson’s mirror galvanometer. This figure 
shows the apparatus in section only, and at c in fig. 153 it is 
seen in elevation. 
The principle of the mirror galvanometer, which is used 
with submarine cable is the same as that of Wheatstone’s 
single-needle instrument. Fig. 153 shows the general arrange. 
Fig. 149. 
ment of the apparatus :— A is the galvanometer, 0 the trans¬ 
mitting key, such as we have already described, L the lamp, 
provided with a tube containing converging lenses, by which 
the ray of light is made to fall upon the mirror in the galvano¬ 
meter, whence it is reflected on to the screen r. The small 
mirror, which is shown of its real size at n s, is hung in a 
tube that slides through the centre of the galvanometer. It 
is attached to this tube above and below by a single fibre of 
cocoon silk. A small magnet is fastened to the back of the 
mirror; and the directive action of a strong curved magnet 
at the top of the case normally keeps the mirror parallel to 
the plane of the apparatus. The light of the lamp, projected 
on the screen as a spot of light, forms the indicator of the 
instrument. 
