The Moving Coil Galvanometer

Introduction 

A galvanometer can be defined as a small device used to detect small amounts of electric current or measure its magnitude. The magnetic needle’s movement determines the current and its intensity. This device was initially discovered in the 1800s, and since then, it has seen several iterations. There are several types of galvanometers, including the Tangent galvanometer, Astatic galvanometer, Mirror galvanometer and Ballistic galvanometer. Presently, only one type of Galvanometer is used widely: the D’Arsonval/Weston type or the moving coil type. 

Today, in this article, we will be talking about one of the most important chapters of Physics called “The Moving Coil Galvanometer.” So, if you’ve any doubts regarding the same or haven’t started yet, it’s the best way to go for it. Read our article till the end and make sure you don’t miss out on any point as each mark matters. Without any further ado, let’s get started. 

Define Galvanometer and its Types 

First discovered in the 1800s, the Galvanometer is a small device commonly found at homes and workspaces used to detect the magnitude of the small electric current. A Galvanometer consists of a small needle and a magnetic coil which indicates the intensity of the electric current. The different types of the Galvanometer are as follows- 

• Tangent Galvanometer 
• Ballistic Galvanometer
• Astatic Galvanometer
• Mirror Galvanometer

What is a Moving Coil Galvanometer? 

A moving coil galvanometer is a sensitive electromagnetic device used to measure the electric current. Even if the current is extremely low, it has the ability to measure it accurately. The Moving Coil Galvanometer is majorly divided into two parts- 

• Weston galvanometer or Pivoted-coil
• Suspended coil galvanometer

Application of the Moving Coil Galvanometer

Since this instrument is extremely sensitive in nature because of which it is appropriate to be used to detect current in any of the circuits. When it is connected to the Wheatstone’s bridge circuit, it shows no current flowing through the device or reflects null deflection. Depending upon the current flowing directions, the pointers move to the right and left. Some of the applications of the Moving Coil Galvanometer are as follows- 

• The Moving Coil Galvanometer is used to identify the circuit current value by connecting it in series to low resistance 
• The Moving Coil Galvanometer is used to measure the voltage.

Merits and Demerits of a Moving Coil Galvanometer

Like every other thing on this planet Earth, the Moving Coil Galvanometer also has advantages as well as disadvantages. Below, we have listed a few of them. Let’s have a look- 

Advantages of the Moving Coil Galvanometer: 

• Remains unaffected by the stray magnetic field
• The Moving Coil Galvanometer is highly sensitive 
• It promotes high reliability and accuracy 
• The weight ratio is higher.

Disadvantages of the Moving Coil Galvanometer: 

• The Moving Coil Galvanometer can only be used to calculate direct currents. 
• While measuring direct current, it could show several errors due to permanent magnets, any inside damage, and ageing of the Galvanometer. 

Now, when you’ve understood the advantages and disadvantages of the Moving Coil Galvanometer, it’s time to move further and describe the working of the Moving Coil Galvanometer. Without any further delay, let’s quickly get started! 

Working of the Moving Coil Galvanometer

Assume ABCD is the single rectangular coil. The current I flow through the coil. If the coil is positioned in a magnetic field donated by B, the rectangular coil feels a torque τ. Let’s consider the rectangular coil ABCD with a breath b and length L. This is positioned in a way that the magnetic field is parallel to the plane of the coil. Since AB and DC are placed parallel to the direction of the magnetic fields, no effective force is experienced at all. Since AB and BC are perpendicular to magnetic field direction, it experiences effective force donated by F.

 F = BIL

According to Fleming’s left-hand rule, it can be said that the forces on BC and AD are not in a similar direction to each other. When opposite or equal forces act on a rectangular coil, it leads to torque, due to which the coil deflects.  

τ = force x perpendicular distance between the forces or 

τ = F × b

If the value of F is substituted, we know 

BIL× b where area A of the coil is L x b, 

Hence it appears to be τ = nIAB

Sensitivity of the Moving Coil Galvanometer

The Sensitivity of the Moving Coil Galvanometer can be defined as the ratio of change in deflection of the Galvanometer to the change in current in the coil. This is donated by S = dθ/dI. Where S is the Sensitivity of the Moving Coil Galvanometer,  θ is deflection in the galvanometer and I is the current. 

The Galvanometer can be termed as sensitive if it gives a larger deflection for a small current. 

Sensitivity of the Moving Coil Galvanometer increases due to the following reasons- 

• Increase in the coil area and the number of the turns and 
• Stronger magnetic induction.

Factors Affecting Sensitivity of the Moving Coil Galvanometer

The following are the major factors affecting the sensitivity of the Moving Coil Galvanometer. Let’s have a look- 

• Increase in the coil area
• Number of coil turns 
• The magnitude of a couple per unit twist k/nAB
• Magnetic strength field B. 

Conclusion 

The Moving Coil Galvanometer can be defined as a small device that is used for detecting small amounts of electric current or measuring its magnitude. The magnetic needle’s movement determines the current and its intensity. 

This device was initially discovered in the 1800s, and since then, it has seen several iterations. The different types of the Galvanometer. These include Tangent galvanometer, Astatic galvanometer, Mirror galvanometer and Ballistic galvanometer. Like every other thing on this planet Earth, the galvanometer has several merits and demerits that we have already discussed above.