A galvanometer has an internal resistance of 28 Ohm and deflects full scale for

Question

A galvanometer has an internal resistance of 28 Ohm and deflects full scale for a 70- mu A current. Describe how to use this galvanometer to make an ammeter to read currents up to 25 A A resistor must be placed in parallel with the galvanometer. A resistor must be placed in series with the galvanometer. What is the magnitude of this resistor? Express your answer using two significant figures. A galvanometer has an internal resistance of 28 Ohm and deflects full scale for a 70- mu A current. Describe how to use this galvanometer to make an voltmeter to give a full-scale deflection of 200V. A resistor must be placed in series with the galvanometer. A resistor must be placed in parallel with the galvanometer. What is the magnitude of this resistor? Express your answer using two significant figures.

Explanation / Answer

The current through galvo at FSD will be

: = 70*10^-6 A

voltage = 70*10^-6*28 = 1.96*10^-3 V

Now, the resistor, R, that we will add should be in series with the galvanometer then it will also pass 70*10^-6A
The voltage that we will supply = 200V we apply, will be across the resistor (R) and galvanometer combination. But, at FSD, there will be only 1.96*10^-3V across galvanometer

leaving only: 200 - 1.96*10^-3 V across R.

Using ohms law R = V/I
R = (200-.00196)/70*10^-6
R = 2857114.85 ohm

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