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, which gives it a long period of oscillation. It is really an integrator that measures the amount of charge discharged through it. It can be the type of moving coil or the type of moving magnet.
Grassot Flowmeter Calibration Arrangement using a standard mutual inductor and a known amount of electrical discharge. The measurement configuration is similar.
Before the 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 deflection. The constant K is calculated from the capacitance C, the voltage V and the deflection d
where K is expressed in coulombs per centimeter.
Beginning / Principle: When a current passes through a coil, freely suspended in a magnetic field, it experiences forces in a direction given by Fleming's left-hand rule.
Building / Construction: It consists of a rectangular coil of thin copper wire wound on a non-metallic ivory frame. It is suspended by means of a bronze phosphor wire between the poles of a powerful horse shoe magnet. A small circular mirror attaches to the suspension cable. The lower end of the coil is connected to a hair. The upper end of the suspension wire and the lower end of the spring are connected to the terminals T1 and T2. A soft cylindrical iron core © is placed symmetrically inside the coil between the magnetic poles which are also made cylindrically. This iron core concentrates the magnetic field and helps produce the radial field.
The B.G. is used to measure the electric charge. The charge has to pass through the coil as quickly as possible and before the coil stars are moved. The coil thus receives a pulse and a shot is recorded. To achieve this result, a coil of high moment of inertia is used so that the period of oscillation of the coil is quite large. The coil oscillations are practically undamped.