Current Sensitivity and Conversion to Ammeter and Voltmeter

A galvanometer is an instrument that spots weak electrical voltages and currents. 

A coil is strung between the concave pole sides of a powerful laminated horseshoe magnet. The coil deflects when an electric current flows across it. A pointer is attached to the coil to measure its deflection (or by using a lamp and scale arrangement). The current passed is proportional to the deflection.

The galvanometer coil has a considerable resistance (about 100 ohms). The galvanometer can only carry a tiny amount of current (1 mA).

Applications of galvanometer

Because the moving coil galvanometer is such a sensitive instrument, it may be used to detect the presence of current in any circuit. 

A galvanometer’s pointer exhibits no deflection when used in a Wheatstone’s bridge circuit. It indicates that no current is flowing through the device. Depending on the current direction, the pointer deflects to the left or right.

You can use the galvanometer to measure the following things:

• You can increase the circuit’s current value by connecting the circuit’s current to low resistance in parallel
• You can increase the voltage by putting it in series with high resistance

Let us talk about the ammeter and voltmeter in detail!

Ammeter

An ammeter is a device (instrument) used to measure significant electrical currents in circuits. You can connect it in series with the circuit whose current you need to measure.

An ammeter must have the following two properties for reliable measurement:

• A negligible resistance (zero in an ideal case)
• There is a lot of current-carrying capacity

You can accomplish the above by parallelly connecting a low resistance with the galvanometer’s coil. This low-resistance parallel circuit is the shunt. The shunt lowers the ammeter’s total resistance and boosts its current-carrying capability.

Voltmeter

A voltmeter is a device (instrument) for determining the difference in electric potential between two locations in a circuit. You should connect the voltmeter in parallel with the circuit branch whose potential you need to measure.

A voltmeter must have the following two characteristics for good measurements:

• A significant amount of resistance (infinite in the ideal case)
• There is a minimal current carrying capacity

You can achieve the above by connecting a high resistance in series with the galvanometer’s coil. The series’ high resistance raises the voltmeter’s total resistance and lowers its current carrying capacity.

Difference between Voltmeter/Ammeter

Let us see the difference between an ammeter and a voltmeter. The latter is a device that measures the difference in electrical potential between two places in a circuit. On the other hand, the former is a device that measures the electric current in a circuit.

Conversion of a Galvanometer into an Ammeter

A galvanometer can detect only modest currents.

A tiny resistance termed shunt resistance (S) is placed in parallel across the galvanometer to measure big currents (to convert into an ammeter). Only a tiny portion of total current Ig flows through the galvanometer for full-scale deflection, with the remainder I-Ig by passing through the shunt S. 

Because G and S are parallel to each other, the difference between them is the same.

Conversion of a Galvanometer into a Voltmeter

A voltmeter is a galvanometer with a high resistance that measures the variance between two locations in an electric circuit. You have to always connect it to the two places parallelly. 

A galvanometer’s resistance is quite low. You can increase the galvanometer’s resistance to expand its measurement range. When connected in parallel to a circuit, it draws a weak current and has little effect on the magnitude of current flowing in the main circuit. As a result, an appropriate high resistance is connected in series with the galvanometer to transform it into a voltmeter.

Pros and Cons of a Moving Coil Galvanometer

Advantages

• Sensitivity is high
• There would be little effect of stray magnetic fields
• The torque-to-weight ratio is extremely high
• High precision and dependability

Disadvantages

• It is only suitable for measuring direct currents
• Errors develop due to causes such as instrument ageing, permanent magnets, and mechanical stress damage to the spring

Key Points

1. A voltmeter is a high-resistance instrument always connected in parallel with the circuit part. It is to compute the potential difference.
2. The resistance of an ideal voltmeter is infinite.
3. To expand the voltmeter’s range by n times, the resistance to be connected in series with the galvanometer is R = (n-1) G.

Conclusion

Because a galvanometer is such a delicate device, it cannot detect large currents. 

An extremely low resistance, known as “shunt” resistance, is connected parallelly to the galvanometer to transform it into an ammeter. The shunt’s value has been modified so that most of the current travels through it. 

In this manner, a galvanometer can be converted into an ammeter, allowing it to measure heavy currents without being entirely deflected. For more information on the use of ammeter and voltmeter, simply refer to the study material.