The
mirror is of film glass silvered, the magnets of hair-spring, and both
together sometimes weigh only one-tenth of a grain. A beam of light is
thrown from a lamp upon the mirror, and reflected by it upon a white
screen or scale a few feet distant, where it forms a bright spot of
light.
When there is no current on the instrument, the spot of light remains
stationary at the zero position on the screen; but the instant a current
traverses the long wire of the coil, the suspended magnets twist
themselves horizontally out of their former position, the mirror is of
course inclined with them, and the beam of light is deflected along the
screen to one side or the other, according to the nature of the current.
If a POSITIVE current--that is to say, a current from the copper pole of
the battery--gives a deflection to the RIGHT of zero, a NEGATIVE
current, or a current from the zinc pole of the battery, will give a
deflection to the left of zero, and VICE VERSA.
The air in the little chamber surrounding the mirror is compressed at
will, so as to act like a cushion, and 'deaden' the movements of the
mirror. The needle is thus prevented from idly swinging about at each
deflection, and the separate signals are rendered abrupt and 'dead
beat,' as it is called.
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