Question

A 100mv, 5mA d’Arsonval movement is to be used in an ammeter whose full-scale reading is 1A. Calculate RA.

(a) 0.7 ohms

(b) 0.5 ohms

(c) 0.1 ohms

(d) 0.2 ohms

I had been asked this question by my college director while I was bunking the class.

This intriguing question comes from Measuring Voltage and Current in section Simple Resistive Circuits of Electric Circuits

Answer 1

Right choice is (c) 0.1 ohms

The explanation is: 5mA is flowing through the coil which implies 995mA are diverted through RA.

                                  V = iR

                                     = 100 * 10^-3

                                     = 995 * 10^-3 RA

                                 RA  = 0.100Ω.