Moving coil galvanometers
A galvanometer is a device used to detect or measure the magnitude of a small electric current. Current and its intensity are usually expressed by the movement of a magnetic needle or coil in a magnetic field, which is an essential part of a galvanometer.
Since its discovery in the 19th century, the galvanometer has seen many iterations. Some of the different types of galvanometer are tangent galvanometer, astatic galvanometer, mirror galvanometer, and ballistic galvanometer. However, the main type of galvanometer widely used today is the D'Arsonval/Weston type or the moving coil type. A galvanometer is essentially a historical name given to a moving coil electric current detector.
What is a moving coil galvanometer?
A moving coil galvanometer is a device used to measure electric currents. It is a sensitive electromagnetic device that can measure small currents down to a few microamperes.
Moving coil galvanometers are mainly divided into two types.
Spiral Coil or Weston Galvanometer
Principle of moving coil galvanometer
A current-carrying coil, when placed in an external magnetic field, experiences a magnetic moment. The angle through which the coil is deflected by the magnetic moment is proportional to the magnitude of the current in the coil.
Construction and Diagram of Moving Coil Galvanometer
A moving coil galvanometer consists of a rectangular coil of several turns, usually made of thin insulation or fine copper wire wound on a metal frame. The coil is free to rotate about a fixed axis. A phosphor-bronze strip connected to a movable torsion head is used to suspend the coil in a uniform radial field.
The important characteristics of the material used to hang the coil are conductivity and a low value of torsional constant. To improve the strength of the magnetic field and direct the field radially, a cylindrical soft iron core is placed symmetrically inside the coil. The lower part of the coil is attached to a phosphor-bronze spring with a small number of turns. The other end of the spring is connected to the mounting screws.
A current I passes through a rectangular coil of n turns and cross-sectional area A. If this coil is placed in a uniform radial magnetic field B, a torque τ is applied to the coil.
Let us first consider one turn of a rectangular coil ABCD of length l and width b. It is suspended in a magnetic field of strength B so that the plane of the coil is parallel to the magnetic field. Since the sides AB and DC are parallel to the direction of the magnetic field, they have no effective force under the influence of the magnetic field. The sides AD and BC are perpendicular to the direction of the field, affecting the effective force F, with F = BIl
Using Fleming's left-hand rule, we can determine that the forces AD and BC are opposite. When equal and opposite forces F, called a couple, act on the coil, it produces a torque. This torque causes the coil to bend.
We know that torque τ = force x perpendicular distance between forces
τ = F × b
Substituting the value of F that we already know,
The torque τ acting on the single-circuit winding ABCD = BIl × b
where lx b is the coil area A,
Thus, the torque acting on n turns of the coil is given by the formula
τ = nIAB
The magnetic torque produced in this way causes the coil to rotate and the phosphor bronze strip to twist. The spring S attached to the coil in turn produces a countermoment or restoring torque kθ, resulting in a uniform angular deflection. In balance,
kθ = nIAB
Here, k is called the torsional constant of the spring (returns a couple per unit of rotation). The deflection or twist θ is measured as a value indicated on the scale by a pointer attached to the suspension wire.
Therefore, θ ∝ I
The quantity nAB/k is constant for a given galvanometer. So it is clear that the deflection of a galvanometer is directly proportional to the current passing through it.
Solved Question: What is the introduction of a cylindrical soft iron core into a moving coil galvanometer?
Solution: A cylindrical soft iron core placed inside the galvanometer increases the strength of the magnetic field and thus improves the sensitivity of the instrument. It also makes the field radial so that the angle between the plane of the coil and the magnetic lines of force remains zero during the rotation of the coil.
Sensitivity of a moving coil galvanometer
A general definition of the sensitivity experienced by a moving coil galvanometer is given as the ratio of the change in galvanometer deflection to the change in coil current.
S = d9/dl
The sensitivity of the galvanometer is greater if the device shows a greater deflection at a lower value of current. There are two types of sensitivity namely current sensitivity and voltage sensitivity. Current sensitivity
The deviation θ per unit current I is called the current sensitivity θ/I
θ/I = nAB/k
The deviation θ per unit of voltage is called voltage sensitivity θ/V. Dividing both sides by V in the equation θ= (nAB / k)I
θ/V= (nAB /V k)I = (nAB / k) (I/V) = (nAB /k) (1/R)
R represents the effective resistance of the circuit.
It is worth noting that voltage sensitivity = current sensitivity/winding resistance. Therefore, if R remains constant, voltage sensitivity ∝ current sensitivity.
Galvanometric Usefulness Profile
It is the ratio of the full-scale deflection current to the number of scales on the instrument scale. It is also the inverse of the current sensitivity of the galvanometer.
Factors affecting the sensitivity of the galvanometer
a) Number of coil turns
b) Area of coil
c) Strength of magnetic field B
d) Couple size per torsional unit k/nAB
Applications of Galvanometer
A moving coil galvanometer is a very sensitive device which allows it to be used to detect current in any circuit. When a galvanometer is connected to a Wheatstone bridge circuit, the pointer of the galvanometer shows zero deflection, i.e. no current passes through the device. The cursor will tilt left or right depending on the direction of the flow.
A galvanometer can be used for measurement
a) The value of the current in the circuit connecting it in parallel with a small resistance.
b) Voltage by connecting it in series with high resistance.
Convert Galvanometer to Ammeter:
A galvanometer is converted into an ammeter by connecting it in parallel with a small resistance called a shunt resistance. A suitable shunt resistor is selected according to the range of the ammeter.
In the given circle
RG - Galvanometric resistance
G - Galvanometer coil
I - total current through the circuit
IG - total current through the galvanometer equal to the full scale reading
Rs - Shunt resistance value
When the current IG passes through the galvanometer, the current through the shunt resistor is given by the formula IS = I - IG. The voltages across the galvanometer and the shunt resistor are the same because of the parallelism of their connection.
Hence RG.IG= (I-IG).Rs
The value of S can be obtained using the above equation.
Convert a Galvanometer to a Voltmeter:
A galvanometer is converted into a voltmeter by connecting it in series with a large resistance. A suitable high resistance is selected according to the range of the voltmeter.
In the given circle
RG = galvanometric resistance
R = high resistance value
G = Galvanometer coil
I = total current through the circuit
IG = total current through galvanometer corresponding to full-scale deflection
V = voltage drop in series connection of galvanometer and high resistance
When a current IG passes through a series combination of a galvanometer and a large resistance R, a voltage drop is obtained in the branch ab
V= RG.IG R.IG
The value of R can be obtained using the above equation.
Solved question: A moving coil galvanometer with a resistance of 100 Ω is used as an ammeter with a resistance of 0.1 Ω. The maximum deflection current of the galvanometer is 100 μA. Find the current in the circuit so that the ammeter shows maximum deflection.
Solution: Given that RG = 100 Ω, Rs = 0.1 Ω, IG = 100 μA
We know that RG.IG= (I- IG).RS
Therefore I = (RG . IG IG.Rs)/RS
I = (1 RG/RS). IG
If the given values are substituted, we get I= 100.1mA
Advantages and Disadvantages of Moving Coil Galvanometer
« Stray magnetic fields are not easily affected
« The torque to weight ratio is high
« High accuracy and reliability
It can only be used to measure direct currents. Errors arise from factors such as instrument aging, permanent magnets, and spring damage due to mechanical stress.