A galvanometer of resistance G can be converted into a voltmeter of range V by connecting in series a high resistance R given by:

1. Galvanometer gives full deflection at current I_g, so V_g = I_g G. 2. To make it read up to V, drop the excess (V − V_g) across an added series R. 3. At full deflection I_g flows through both, so I_g R = V − I_g G ⇒ R = V/I_g − G. 4. R is large because V/I_g ≫ G typically. _Source: syllabus Physics Ch 3 §3.10 Galvanometer → Voltmeter_