What is Ballistic Galvanometer : Working & Its Uses

The former galvanometer was introduced by Johann Schweigger in the year 1820. The development of the device was also done by Andre Marie Ampere. The former designs enhanced the effect of the magnetic field that was developed by the current through many numbers of wire turns. So, these devices were also called as multipliers as because of their almost similar construction. But the term galvanometer was more in popularity by 1836. Then after with many enhancements and progressions, various types of galvanometers came into existence. And the one type is “Ballistic Galvanometer”. This article clearly explains its working principle, construction, applications, and advantages.

What is Ballistic Galvanometer?

Ballistic Galvanometer is the device that is employed for assessing the amount of charge flow that is developed from the magnetic flux. This device is a kind of sensitive galvanometer which is also termed as a mirror galvanometer. In contrast to the general kind of measuring galvanometer, the moving section of the device holds a more inertial moment, so it provides a long time of oscillation. It genuinely operates as an integrator calculating the amount of charge expelled from it. This might be like either a moving magnet or like moving coil.

Working Principle

The principle behind the ballistic galvanometer working is that it measures the amount of charge that flows across the magnetic coil where this initiates the coil to move. When there is charge flow across the coil, it provides an increase in the current value because of the torque that is generated in the coil, and this developed torque operates for a shorter period of time.

Ballistic Galvanometer Construction
Ballistic Galvanometer Construction

The result of time and the torque gives force for the coil and then the coil gets is rotating motion. When the starting kinetic energy of the coil is totally employed for operation, then the coil will start out to get to its actual position. So, the coil swings in the magnetic arena, and the deflection is then stated down from where the charge might be measured. So, the principle of the device is mainly dependent on the coil deflection that has a direct relation to the amount of charge that flows through it.

Ballistic Galvanometer Construction

The construction of a ballistic galvanometer is the same as like moving coil galvanometer and it includes two properties where those are:

  • The device has undamped oscillations
  • It also has exceptionally minimal electromagnetic damping

The ballistic galvanometer is included with copper wire where it is rolled across the non-conducting frame of the device. The phosphorus bronze in the galvanometer halts the coil which is present in between the magnetic poles. For the enhancement of magnetic flux, the iron core is placed inside the coil.


The coil’s underneath section is connected with the spring where it gives restoration torque for the coil. When there is charge flow across the ballistic galvanometer, then the coil gets to have a movement and develops an impulse. The coil’s impulse has a direct relation to the flow of charge. The accurate reading in the device is achieved by implementing a coil that holds increased inertial moment.

The moment of inertia implies that the body is in opposition to that of angular movement. When there is increased inertial moment in the coil, then the oscillations will be more. So, because of this precise reading can be achieved.

Detailed Theory

The detailed theory of the ballistic galvanometer can be explained with the following equations. By considering the below example, the theory can be known.

Let us consider a rectangular-shaped coil that has ‘N’ number of turns which is kept in a constant magnetic field. For the coil, the length and breadth are ‘l’ and ‘b’. So, the area of the coil is

A = l × b

When there is current flow across the coil, then the torque is developed on it. The magnitude of the torque is given by τ = NiBA

Let us assume that the flow of current across the coil for each minimal time period is dt and so the change in current is represented as

τ dt = NiBA dt

When there is current flow across the coil for a time period of ‘t’ seconds, then the value is represented as

ʃ0t τ dt = NBA ʃ0t idt  = NBAq

where ‘q’ is the total amount of charge that flows across the coil. The inertial moment that exists for the coil is shown as ‘I’ and the coil’s angular velocity is shown as ‘ω’. The below expression provides the angular momentum of the coil and it is lω. It is similar to the pressure that is applied to the coil. By multiplying the above two equations, we get

lw = NBAq

Also, the kinetic energy across the coil will have deflection at ‘ϴ’ angle and the deflection will be restored using the spring. It is represented by

Restoring torque value = (1/2)cϴ2

Kinetic energy value = (1/2)lw2

As the coil’s restoring torque is similar to the deflection then

(1/2)cϴ2 = (1/2)lw2

2 = lw2

Also, the periodic oscillations of the coil is shown as below

T = 2∏√(l/c)

T2 = (4∏2l/c)

(T2/4∏2) = (l/c)

(cT2/4∏2) = l

Finally, (ctϴ/2∏) =lw = NBAq

q = (ctϴ)/NBA2∏

q = [(ct)/NBA2∏] * ϴ)

Assume that k = [(ct)/NBA2∏

Then q = k ϴ

So, ‘k’ is the constant term of the ballistic galvanometer.

Galvanometer Calibration

The calibration of the galvanometer is the approach of knowing the device’s constant value with the assistance of some practical methodologies. Here are the two methods of the ballistic galvanometer and those are

Calibration using Capacitor

The constant value of the ballistic galvanometer is known with the charging and discharging values of the capacitor. The below ballistic galvanometer diagram using a capacitor shows the construction of this method.

Calibration Using Capacitor
Calibration Using Capacitor

The construction is included with an unknown electromotive force ‘E’ and a pole switch ‘S’. When the switch gets connected to the second terminal, then the capacitor moves to the charging position. In the same way, when the switch gets connected to the first terminal, then the capacitor moves to the discharging position using the resistor ‘R’ that is in series connection to the galvanometer. This discharging causes deflection in the coil at the ‘ϴ’ angle. With the below formula, galvanometer constant can be known and it is

Kq = (Q/ϴ1) = CE/ ϴ1 measured in coulombs per radian.

Calibration using Mutual Inductance

This method needs primary and secondary coils and the galvanometers constant calculates the mutual inductance of the coils. The first coil gets energized through the known voltage source. Due to the mutual inductance, there will be the development of current is the second circuit and this is utilized for the galvanometer’s calibration.

Calibration Using Mutual Induction
Calibration Using Mutual Induction

Ballistic Galvanometer Applications

Few of the applications are:

  • Employed in control systems
  • Used in laser displays, and laser engraving
  • Utilized for knowing photoresistor measurements in the metering method of film cameras.

So, this is all about the detailed concept of a ballistic galvanometer. It explains clearly the device working, construction, calibration, applications, and diagram. It is also more important to know about what the types are in ballistic galvanometer and ballistic galvanometer advantages?

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