Question

The resistance required to be connected in parallel to an ammeter in order to increase its range 10 times, will be

• A. one-tenth the resistance of ammeter
• B. nine times the resistance of ammeter
• C. ten times the resistance of ammeter
• D. one-ninth the resistance of ammeter

Answer 1

The correct option is D one-ninth the resistance of ammeter

Full scale deflection voltage be $V$
Now initiall range is $I$
$V=IR$
Now, Range is made $10I$
$V=10I{R}^{\prime }$
$R^{\prime }=V/10I=R/10$
therefore, new$R_{eq}$ of parallel combination will be R/10
therefore,
$1/R_{eq}=1/R+1/R_p$
$1/R_p=1/R_{eq}-1/R=10/R-1/R=9/R$
therefore, $R_p=R/9$
option(D)