A circuit you\'re building needs an ammeter that goes from 0 mA to a full-scale

Question

A circuit you're building needs an ammeter that goes from 0 mA to a full-scale reading of 30.0 mA. Unfortunately, the only ammeter in the storeroom goes from 0 microA to a full-scale reading of 500 microA only . Fortunately, you've just finished a physics class, and you realize that you can make this ammeter work by putting a resistor in parallel with it, as shown in the figure:

http://session.masteringphysics.com/problemAsset/1074658/3/knight_Figure_31_65.jpg

You've measured that the resistance of the ammeter is 90.0 ohms,
not the 0 ohms of an ideal ammeter.

What is the affective resistance of your ammeter?

Explanation / Answer

With the resistance in parallel current is split so wen need that resis tance to draw the current of 30*10^-3 -500*10^-6 =0.0295 Let its value be R current drawn = 30*10^-3 *90/(90+R) =0.0295 solving we get R=1.5254 ohms so Reff = 90*R/(90+R)=1.50ohm

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