ELECTEICAL PEINCIPLES OP THE ATLANTIC TELEGEAPH. 421 
being fastened together with their similar poles pointing in 
opposite directions by a thin strip of aluminium. Fastened to 
the upper magnet (the one inside the bobbin A, fig. 1) is a 
circular mirror of silvered glass, about | of an inch in diameter, 
and weighing less than a grain. This mirror is ground 
slightly concave; so that, when the light of a paraffin lamp 
shines upon it through the opening’ C (fig. 1), an image of the 
opening is formed upon the scale D E at a distance of about 
three feet.* The position of the magnets and mirror inside 
the bobbin is indicated at M and N, fig. 2, and they are repre- 
sented separately and of their real size in fig. 3. A curved 
steel magnet, F (fig. 1), about eight inches long, which can be 
placed at any height upon the vertical brass rod which supports 
it, and can be turned round this rod as an axis, either by hand 
or by the adjusting- screw Gr, serves to neutralise to any 
required extent the effect of the eartJFs directive force upon 
the suspended magnets, and to cause them to hang, when no 
current is passing through the instrument, so that the axis of 
the little mirror coincides with that of the upper bobbin. 
Under these circumstances, the image of the slit G is formed 
at the middle of the scale D E, and appears as a bright spot or 
line of light rendered more visible by the shadow cast upon 
the scale by the projecting screen above it. There are several 
other points of considerable interest in the construction of 
this apparatus, but we have not space to dwell upon them. 
The simplest mode of using this apparatus, for the purpose 
of receiving telegraphic signals, is to connect one of the 
spirally coiled wires, seen at the lower part of the drawing, 
with the conductor of the cable, and to connect the other 
with the ground. The electric current, before it can pass from 
the cable into the earth, is thus obliged to traverse the whole 
length of wire forming the coils of the galvanometer, and in 
doing so it causes the little magnets (which in their natural 
position hang with their axes at right angles to the axes of the 
coils) to turn either towards the right or left, carrying with 
them the mirror, and thus causing the spot of light to move 
along the scale towards one end or the other. The stronger 
the current which is passing through the galvanometer, the 
further does the spot of light travel ; and if the current con- 
tinues of the same strength for any length of time, the spot of 
light comes to rest at a corresponding distance from the centre 
of the scale. Thus every variation in the strength of the 
* The use of a concave mirror in these instruments is a very recent 
improvement. Until lately a flat mirror was employed, and a small convex 
lense, placed close in front of it, caused the rays which issued from the 
opening C to he brought to a focus upon the scale after reflection by the mirror. 
