Referring to the cut, Fig. 179, M is the suspended magnet, with north
and south poles n and s; S is the reflecting mirror; r is the tube
containing the suspending thread; R is the damper removed for ballistic
work.
The ballistic galvanometer is used to measure quantities of electricity
in an instantaneous discharge, which discharge should be completed
before the heavy needle begins to move. The extreme elongation or throw
of the needle is observed, and depends (1) on the number of coulombs (K)
that pass during the discharge; (2) on the moment of inertia of the
needle and attached parts; (3) on the moment of the controlling forces,
i. e., the forces tending to pull the needle back to zero; (4) on the
moment of the damping forces; (5) on the moment of the deflecting forces
due to a given constant current. The formula is thus expressed: