What is "ballistic galvanometer"

n. a moving-coil galvanometer that measures electric charge

A ballistic galvanometer is a type of sensitive galvanometer, commonly a mirror galvanometer. Unlike a current-measuring galvanometer, the moving part has a large moment of inertia, thus giving it a long oscillation period. It is really an integrator measuring the quantity of charge discharged through it. It can be either of the moving coil or moving magnet type.

Before first use the ballistic constant of the galvanometer must be determined. This is usually done by connecting to the galvanometer a known capacitor, charged to a known voltage and recording the deflection. The constant K is calculated from the capacitance C, the voltage V and the deflection d: K = CV/d

where K is expressed in coulombs per centimeter.

In operation the unknown quantity of charge Q (in coulombs) is simply: Q = kd.