Ammeter and voltmeter

An ammeter is an electrical device that is used to measure the current. It is the modified form of the galvanometer. It is a low-resistance galvanometer. It is always connected in series with the circuit to measure current. An ideal ammeter has zero resistance. While Voltmeter is an electrical device that is used to measure the potential difference between two points.

Meter movement

The portion of the galvanometer whose motion causes the needle of the device to move across the scale is usually known as meter-movement,
Mostly, the galvanometer gives full-scale deflection even when a few milliamperes current passes through it. So for large current measurements, it should be modified.
See also: Series and Parallel Circuits

Conservation of galvanometer into Ammeter

Shunt (bypass) resistor
A galvanometer can be converted into an ammeter by connecting a low resistance R_s called shunt (bypass) resistor in parallel with the meter-movement (coil) of the galvanometer.
Range of Ammeter
Let, I be the range of the ammeter.
See also: Emf Vs P.D
Calculation of Shunt Resistance
Shunt resistance provides another path for the flow of current. The value of R_s is so adjusted that the current which gives full-scale deflection passes through the galvanometer and the remaining current passes through the R_s.
Let
The resistance of galvanometer = R_g
Full-scale deflection current = I_g
The current passing through R_s = I – I_g
Now, the potential difference across the R_g is:

V_g = I_g R_g  ————-  (1)

Similarly, the potential difference across the R_s is:

V_s = I_g R_s
V_s = (I – I_g) R_s  ————  (2)

Since the galvanometer resistance, R_g, and the shunt resistance R_s are connected in parallel, so the potential difference between them must be the same i.e.,

V_s = V_g

Putting values, we get:

(I – I_g) R_s  = I_g R_g
Or
R_s  =  I_g R_g/ (I – I_g) —————-  (3)

The value of the shunt R_s is very small. We can use copper wire for this purpose which should be connected in parallel across the galvanometer. In order to increase the range of the ammeter, R_s should be decreased.
See also: Resistance

What is voltmeter?

The voltmeter is an electrical device that is used to measure the potential difference between two points. it is the modified form of the galvanometer. It is a high-resistance galvanometer. It is always connected in parallel with the circuit component. An ideal voltmeter has infinite resistance.

Conservation of galvanometer into Voltmeter

High Resistance
A galvanometer can be converted into a voltmeter by connecting a very high resistance R_h in series with meter-movement (coil) of the galvanometer.
Range of voltmeter
Let, the range of the voltmeter n= V (volts)
Calculation of High resistance (R_h)
The value of the high resistance R_h should be such that the galvanometer gives full-scale deflection when it is connected to V volt.
Let,
The resistance of meter-movement = R_g
Full scale deflection current = I_g
Since R_g and R_h are in series, so
The combined resistance of the meter movement and R_h = R_g + R_h
Applying Ohm’s law, we have
V = I_g (R_g + R_h)
V = I_g R_g+ I_g R_h
Or           I_g R_h = V – I_g R_g
Or        R_h = V/I_g – R_g
This equation gives the value of the high resistance R_h which is used to convert the galvanometer into the voltmeter. The scale is calibrated from 0 to V to measure the voltage. In order to increase the range of the voltmeter, R_h should be increased.
The resistance of the voltmeter should be greater than the circuit resistance otherwise it will change the value of potential difference which is to be measured.
Related Topics:

• Difference between current and voltage
• Ohm’s law formula
• Types of charges
• Coulomb’s law formula
• Gauss’s law
• Difference between conductors and insulators