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A 50 ohm resistor is connected across a 5 V battery. To measure the current through this resistor, a galvanometer of resistance 100 ohm is converted into an ammeter by attaching an extra resistance rₛ. Which arrangement keeps the measured current within 1% of the true (no-ammeter) value?

1. rₛ = 1 ohm in series with the galvanometer
2. rₛ = 0.5 ohm in parallel with the galvanometer
3. rₛ = 0.5 ohm in series with the galvanometer
4. rₛ = 1 ohm in parallel with the galvanometer

Correct answer: rₛ = 0.5 ohm in parallel with the galvanometer

Solution

A 100 ohm galvanometer can read the line current only if a small parallel shunt drops its effective resistance to ~0.5 ohm, keeping the added resistance below the 1% tolerance limit.

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