Introduction:
This question is related to the measurement of current using a moving coil instrument. The instrument has a full-scale deflection of 1 mA and a resistance of 5 ohms. In addition, a resistance of 0.55 ohms is connected in parallel to the instrument.

Calculation:
The maximum current that can be measured by the instrument can be calculated using the following formula:

Imax = IFS * R / (R + Rp)

Where Imax is the maximum current that can be measured, IFS is the full-scale deflection current (1 mA), R is the resistance of the instrument (5 ohms), and Rp is the parallel resistance (0.55 ohms).

Substituting the values in the above formula, we get:

Imax = 1 mA * 5 ohms / (5 ohms + 0.55 ohms)
Imax = 0.909 mA

Therefore, the maximum current that can be measured by the instrument is 0.909 mA.

Explanation:
The moving coil instrument works on the principle of electromagnetic induction. When a current flows through the coil, it produces a magnetic field, which interacts with the magnetic field of a permanent magnet, causing the coil to rotate. The amount of rotation is proportional to the current flowing through the coil.

The instrument has a full-scale deflection of 1 mA, which means that when a current of 1 mA flows through the coil, it produces a maximum rotation of the coil. The resistance of the instrument is 5 ohms, which means that the current flowing through the coil is limited to 1 mA.

When a resistance of 0.55 ohms is connected in parallel to the instrument, it creates a parallel circuit. This reduces the effective resistance of the circuit, which increases the current flowing through the circuit. The maximum current that can be measured by the instrument is limited by the resistance of the instrument and the parallel resistance.

Therefore, by calculating the maximum current that can be measured using the formula, we can determine the maximum current that can be measured by the instrument.