Ballistic Galvanometer – Working Principles and Uses

The working principles and uses of a ballistic galvanometer are vividly explained in this post. A ballistic galvanometer is a two-coil moving coil instrument which serves both as an ammeter and voltmeter. Its primary coil is powered by a known voltage source while mutual induction causes current to enter its secondary circuit through mutual inductance.

Suspension wire for coil is made up of phosphor bronze which allows it to remain suspended easily and allows a mirror to be placed below pointer for parallax error correction.

Deflection of the Coil of a Ballistic Galvanometer

A rectangular coil with n turns and cross-sectional area A is immersed in an even, uniform magnetic field B, then when current flows through it it experiences torque proportional to its deflection – this deflection being measured using a plane mirror attached to its suspension wire along with lamp and scale arrangements; its restoring force being provided by a spring.

A high moment of inertia coil wound on non-conductive frame to avoid electromagnetic damping is utilized for creating high precision ballistic galvanometer.

This device is known as a fast reacting galvanometer due to its rapid upswing response time. As such, it makes for an excellent meter choice over PMMC (Permanent Magnet Moving Coil) galvanometers, while boasting linear scale readout. Unfortunately though, they’re not suitable for measuring AC voltage levels.

Deflection of the Pointer of a Ballistic Galvanometer

A ballistic galvanometer is a type of sensitive galvanometer with zero damping torque. The coil in this instrument consists of copper wire wound onto an non-conducting frame. A phosphorous bronze suspension suspends this coil between the north and south poles of a magnet for increasing magnetic flux; at its base is connected by spring which provides restoring torque.

As electricity flows through a coil, it deflects its pointer. The magnitude of this deflection depends on how much electricity passes quickly through it.

Ballistic galvanometers offer many advantages and uses, including their linear scale and accuracy. Unfortunately, however, their use may also lead to errors due to ageing components like springs and permanent magnets, interference from stray magnetic fields and induced voltages as well as keeping away from AC sources. Calibration can be performed to establish their meter constants by discharging different capacitors through it while measuring their swing against how much current has passed through their coil.

Deflection of the Mirror of a Ballistic Galvanometer

The ballistic galvanometer differs from its moving coil counterpart in that it doesn’t measure current, but rather measures most of the charge passing through magnetic fields. Its working principle relies on Fleming’s left hand rule; deflection of its coil is proportional to any impulse exerted by minstrument comprises of a coil of copper wire with large moment inertia wrapped on non-conductive phosphorous bronze and suspended between the north and south poles of a magnet. Due to this non-conductive frame, there are no EMF emissions nor electromagnetic damping effects; thus creating an electromagnetic stability of zero.

Galvanometers rely on two coils, with one primary and one secondary coils connected by mutual inductance. A known voltage source energizes the primary coil which in turn induces current in the secondary coil through mutual induction; this current is then used for calibration. Furthermore, this arrangement acts as a constant for ballistic galvanometers due to mutual inductance of both coils.

Deflection of the Hairsprings of a Ballistic Galvanometer

Deflection of the pointer can be adjusted by altering the torsional head or spring attached to the suspension wire, enabling use as a DC current meter. A ballistic galvanometer’s pointer movement is highly damped compared to regular ammeters which often swing violently when current changes.

The suspension wire is connected to a soft iron core which gives the galvanometer its restoring torque. A phosphorous bronze suspension frame supports its coil and small weights are added for increased inertia.

This type of galvanometer is highly sensitive, measuring currents as low as microamperes. However, it should only be used for DC measurements due to its sensitive components like springs and permanent magnets becoming faulty over time. Utilizing the moving-coil principle and employing two-pole switch S and an unknown EMF source as components age. In its operation it utilizes capacitor discharge through resistor R connected series with galvanometer whose mutual inductance determines its constant.

The charge which flows round a given circuit is directly proportional to the change of flux linkage. If the charge flowing is measured by a ballistic galvanometer G, then we have a measure of the change in flux linkage, ɸ.

Ballistics is the study of the motion of a body, such as a projectile, which is set off by a blow, and then allowed to move freely. By freely, we mean without friction. A ballistic galvanometer is one used to measure an electrical blow, or impulse: for example, the charge Q which circulates when a capacitor is discharged through it.

Working Principle of a Ballistic Galvanometer

A galvanometer which is intended to be used ballistically has a heavier coil than one which is not; and it has as little damping as possible —- an insulating former, no short-circuited turns, no shunt. The mass of its coil makes it swing slowly; for instance, when a capacitor is discharged, and the charge has finished circulating, while the galvanometer coil is just beginning to turn. The galvanometer coil continues to turn however; and as it does so it twists the suspension. The coil stops turning when its kinetic energy, which it gained from the forces set up by the current, has been converted into potential energy of the suspending fibre. The coil then swings back, as the suspension untwists itself, and it continues to swing back and forth for some time. Eventually it comes to rest, but only because of the damping due to the viscosity of the air, and to the internal friction of the fibre. Theory shows that, if the damping is negligible, the first deflection of the galvanometer is proportional to the quantity of electricity, Q,  that pass through its coil, as it began to move. This first deflection Ɵ is often called the ‘throw’ of  the galvanometer; we have then,

Q=kƟ,

Where k is a constant of the galvanometer.

The equation above is true only if all the energy given to the coil is spent in twisting the suspension. If an appreciable amount of energy is used to overcome damping ‑‑‑‑i.e dissipated as heat by eddy currents —then the galvanometer is not ballistic, and Ɵ is not proportional to Q.

To calibrate the ballistic galvanometer, a capacitor of known capacitance, e.g. 2 µF, is charged by a battery of known e.m.f., e.g. 50 volt, and ten discharged through the instrument. Suppose the deflection is 200 divisions. The charge Q=CV = 100 microcoulomb, and thus the galvanometer sensitivity is 2 divisions per microcoulomb.

Working principles and uses of a ballistic galvanometer in video