Measurement of Current using a Moving Coil Millimeter

Introduction
In electrical engineering, a moving coil millimeter is a commonly used instrument to measure electrical quantities such as current, voltage, and resistance. It operates on the principle of electromagnetic induction and consists of a coil of wire suspended in a magnetic field. When a current passes through the coil, it experiences a torque that causes it to rotate. The deflection of the coil is proportional to the current passing through it.

Given Information
- Resistance of the moving coil millimeter: 10 ohms
- Full-scale deflection current: 5 mA

Explanation
To measure currents up to 1A, we need to select an appropriate range and shunt resistance for the moving coil millimeter.

Shunt Resistance Calculation
The shunt resistance is connected in parallel with the moving coil millimeter to divert a portion of the current away from the coil. This allows us to measure higher currents without overloading the instrument. The shunt resistance can be calculated using the formula:

Rshunt = (Rmeter / Ifsd) - Rmeter

Where:
- Rshunt is the shunt resistance
- Rmeter is the resistance of the moving coil millimeter
- Ifsd is the full-scale deflection current

Substituting the given values:
Rshunt = (10 / 0.005) - 10
Rshunt = 2000 - 10
Rshunt = 1990 ohms

Therefore, a shunt resistance of 1990 ohms should be connected in parallel with the moving coil millimeter to measure currents up to 1A.

Range Selection
The range of the moving coil millimeter is determined by the maximum current that can be measured without causing damage to the instrument. In this case, the maximum current is 1A.

Final Setup
To measure currents up to 1A using the moving coil millimeter, the following setup is required:
- Connect the shunt resistance of 1990 ohms in parallel with the moving coil millimeter.
- Connect the positive terminal of the current source to one end of the shunt resistance and the negative terminal to the other end.
- Connect the positive terminal of the moving coil millimeter to the junction of the shunt resistance and current source.
- Connect the negative terminal of the moving coil millimeter to the negative terminal of the current source.

Conclusion
By using a shunt resistance of 1990 ohms and appropriate range selection, the moving coil millimeter can be used to measure currents up to 1A accurately. The shunt resistance diverts a portion of the current away from the coil, preventing damage to the instrument and ensuring reliable measurements.