Question

A voltmeter of variable ranges 3V, 15C, 150V is to be designed by connecting resistances R₁, R₂, R₃ in series with a galvanometer of resistance G = 20Ω, as shown in Fig. The galvanometer gives full pass through its coil. Then, the resistances R₁, R₂ and R₃ (in kilo ohms) should be, respectively –

• A. 3, 12, 135
• B. 2.98, 12, 135
• C. 2.98, 14.98, 149.98
• D. None of these

Answer 1

Answer: (B)

2.98, 12, 135

Answer 2

R = $\frac{V}{I_{g}}$ – R_g

R₁ = $\frac{3}{I_{g}}$ – R_g = 2980Ω = 2.98kΩ

(R₁ + R₂) = $\frac{15}{I_{g}}$ – R_g = 14980Ω = 14.98kΩ

(R₁ + R₂ + R₃) = $\frac{150}{I_{g}}$ – R_g = 149980 Ω

= 149.98 kΩ

⇒ R₁ = 2.98 kΩ

R₂ = 14.98 – 2.98 = 12 kΩ

R₃ = 149.98 – 14.98 = 135 kΩ