Question

How will you use a P.M.M.C. instrument which gives full scale deflection at 50 mV p.d. and 10 mA current as 

(1) Ammeter 0 - 10 A range 

(2) Voltmeter 0-250 V range

Answer 1

Resistance of the instrument R_m = 50 mV/10 mA = 5

(i) As Ammeter 

full-scale meter current, I_m = 10 mA = 0.01 A 

shunt current I_s = I −I_m = 10 − 0.01 = 9.99 A 

Reqd. shunt resistance, S = (I_m.R_m)/(I - I_m) = (0.01 x 5)/9.99 = 0.0005 Ω 

(ii) As Voltmeter

Full-scale deflection voltage, ν = 50 mV = 0.05 V; V = 250 V

Reqd. series resistance, R = (V - v)/I_m = (250 - 0.05)/0.01 = 24,995 Ω