Question

A galvanometer of resistance $ 100\,\Omega $ gives full scale deflection for 20 mV. Find the resistance to be attached so that it gives full scale deflection of 5 V:

• A. $ 24.9\times {{10}^{3}}\,\Omega $ in series
• B. $ 24.9\times {{10}^{3}}\,\Omega $ in parallel
• C. $ 44.9\times {{10}^{3}}\,\Omega $ in series
• D. $ 44.9\times {{10}^{3}}\,\Omega $ in parallel

Answer 1

Answer: (B)

For full scale deflection for 20 mV.
$ i=\frac{20mV}{100\,\Omega }=0.2\,mA $ Again for full scale deflector of 5 V.
$ (R+100)\,i=5\,V $ or $ R+100=\frac{5}{i}=\frac{5\,V}{0.2\,mA} $ $ =25\times {{10}^{3}}\Omega $ $ R=2500-100 $ $ 24900=24.9\times {{10}^{3}}\,\Omega $