working of a permanent magnet moving coil Glavanometer

Have you ever wondered how to measure small electric currents accurately? Look no further than the moving coil galvanometer. Invented in the 19th century, this device uses electromagnetic principles to detect and measure tiny currents. In this article, we will dive deep into the workings of a moving coil galvanometer, exploring its history, construction, and applications. Whether you're a student of physics or just curious about how this essential instrument works, we've got you covered. So, join us on this journey to unlock the mysteries of the moving coil galvanometer and discover its vital role in modern science and technology.

What is Galvanometer ?

Galvanometer is an Electromechanical instrument, used for detecting and indicating an electric current. It is a sensitive electromagnetic device which can measure low currents, even of the order of a few micro-amperes.

Galvanometer works as an actuator, by producing a rotary deflection in response to electric current flowing through a coil in a constant magnetic field.

1. Constructional detail of permanen
t magnet moving coil Galvanometer.

• 
  Permanent magnet moving coil galvanometer is made up of a two concave permanent magnet, where magnetic field is directed from north to south. 
• And rectangular coil that has many turns usually made of thinly insulated or fine copper wire,
• Is wounded on a Metallic frame, which is pivoted from both the end, the coil is free to rotate about a fixed axis,
• This whole arrangement consists of a two phosphor bronze hair like springs, the first one is connected to the upper pivoted free end of the coil and second spring is connected to the bottom pivoted free end of the coil.
• And both the springs are fixed from the other end with the help of a Binding screw.
• A cylindrical soft iron core is symmetrically positioned inside the coil, to increase the strength of the magnetic field and make the field radial.

"There is a very important reason for keeping the magnetic field radial which we will talk about later in this Article during the working of a galvanometer"

• Also the Pointer is mounted on a pivoted coil for measuring the angular deflection in the coil.
• And two end of the coil is connected to the two terminals of the galvanometer

2. Principle of Permanent Magnet moving Coil Galvanometer (PMMCG)

Principle- Permanent magnet moving coil galvanometer works on the principle that current carrying coil placed in a magnetic field experiences a torque.

Working- When the current is allowed to pass through the coil, current starts flowing from A to B in the left arm of the coil and C to D in the right arm of the coil. As we can see in the animation that in the arm AB direction of the current is upward and in the arm CD direction of the current is downward, while the direction of magnetic field is from north to south for both the arm AB and CD. Therefore according to the left hand Fleming rule two couple of forces or torque starts acting on a coil Which will help in rotating of a coil, and causes angular  deflection of the pointer (at this time hair spring comes into play and creates counter torque to resist any unnecessary deflection or damage of a very sensitive pointer and always ensures proper functioning of the galvanometer).

Corresponding calibrated scale is provided for converting the angular deflection of a pointer in micro amperes.

3. Why radial magnetic field is needed ?

Here we have used concave shape magnets to provide radial magnetic field, But why ? lets expore it. 

As we all know equation of torque in this case is-

T = N I A B sin𝚹

Where-

N = Number of turns

I =  Current 

𝚹 = Angle Between B and A ( B - magnetic field , A- Area Vector )

Note :- Area vector always normal to the plane of a rectangular coil

Its very clear that,

Sin 90 = 1 (unity)

lets move to the top view and try to analyze It,

now we can see that because of the radial magnetic field for each and every positons of the coil magnetic field and area vector are at right angle or 𝚹= 90°,

In this way radial magnetic field provides constant torque irrespective of the rotation of the coil -

This makes the deflection directly proportional to the current

because

N A B was already constant and now 𝚹= 90 (which is also constant) 
thats why now 
I - is the only variable.

eq.- T= K I ( K= is for constants )

which makes it easy to calibrate the instrument for the Measuring purpose. 🙂

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