Galvanometer Resistance by Half Deflection
In the half-deflection method, a shunt S is connected across a galvanometer of resistance G while the source and series resistance remain fixed; circuit current division gives G exactly, with G approximately S only under the usual large-series-resistance condition.
Why this shows up in the exam
Measuring galvanometer coil resistance · Designing shunts · Checking validity of half-deflection observations
Learn the idea
A shunt that halves galvanometer deflection allows its resistance to be inferred from the changed current division. Deflection tracks galvanometer current. Adding a shunt diverts current, and the half-deflection condition provides a measurable balance, but the simple G equals S shortcut needs the series resistance to be very large.
🧠 Memory hook: Half deflection makes the shunt nearly equal to G only when series R is large.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- G = RS/(R-S) — common ideal-source half-deflection result for series resistance R in the standard circuit
- G approximately S when R is much greater than G and S — large-series-resistance approximation
How to approach it
- 1Draw the before-and-after circuits
- 2Relate deflection to galvanometer current
- 3Use the exact relation unless the large-R approximation is justified
Common slip-ups that cost marks
- •Stating G always equals S exactly
- •Changing the source or series resistor between readings
- •Confusing half current with half total circuit current
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In a meter bridge, resistance 2 ohm is in the left gap and balance occurs at 40 cm from the left end. Find the right-gap resistance.
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