To convert a milli-ammeter to an ammeter, a resistor of low value is necessary to connect a resistor of

Analyze the question: A milli-ammeter can only carry a small current through its coil. To make it measure larger currents (an ammeter), we must extend its range. The question is asking what kind of resistor to add, and how to connect it.

Concept: To extend the range of a galvanometer or milli-ammeter, we connect a shunt. A shunt is a resistor of very low resistance connected in parallel with the meter.

Why this works:

• In a parallel connection, current divides between the branches. The branch with the lower resistance takes the larger share of the current.
• The low-resistance shunt carries most of the current, so only a small, safe fraction passes through the milli-ammeter coil.
• The total current the combination can measure is therefore much larger.
• The combined resistance of the meter and shunt is also very small, so the ammeter has little effect on the circuit it is placed in. This is what an ideal ammeter should have.

Shunt formula:

R_s = I_g R_g / (I − I_g)

where I_g is the full-scale current of the milli-ammeter, R_g is its resistance, and I is the new maximum current. Since I is much larger than I_g, R_s comes out very small.

Checking the options:

• A. Low value in series: this adds resistance in line with the meter and does not divert any current. Wrong.
• B. High value in series: this is used to make a voltmeter (a multiplier). Wrong.
• C. Low value in parallel: this is a shunt. Correct.
• D. High value in parallel: almost all the current would still go through the meter and damage it. Wrong.

Answer: C. low value in parallel with it

Exam tip: Remember it as follows:

• Ammeter = Amall (small) resistance in parallel (shunt)
• Voltmeter = Very large resistance in series (multiplier)