If the mirror swings through a horizontal arc, the reflected image will
move, in virtue of a simple geometrical principle, through an arc of
twice as many degrees. The scale can be placed far from the mirror, so
that the ray of light will represent a weightless index of very great
length, and minute deflections of the needle will be shown distinctly
upon the scale.
In the cut, Fig. 182, the ray of light from the lamp passes through the
aperture, m m, and is made parallel by the lens, L. At s is the mirror
attached to the needle and moving with it. A scale placed at t receives
the reflection from the mirror. The cut, Fig. 183, shows one form of the
instrument set up for use.
Synonym--Mirror Galvanometer.
Galvanometer Shunt.
To prevent too much current passing through a galvanometer (for fear of
injury to its insulation) a shunt is sometimes placed in parallel with
it. The total current will be distributed between galvanometer and shunt
in the inverse ratio of their respective resistances. (See Multiplying
Power of a Shunt.)