

Voltmeter Introduction A voltmeter measures the potential difference between any two points of a current carrying circuit (or b/w the two terminals of a source). Principle and Construction A voltmeter is always placed in parallel with resistance or apparatus across which the potential difference has to be measured. Voltmeters ought therefore to have a high resistance compared with the resistance across which the voltage is to be measured, so that they take a comparatively negligible current and so disturb the circuit as little as possible. Working For measuring a P.d much higher than its small range, the galvanometer will be unsuitable thus for measuring the P.d accurately without affecting it in any way the voltmeter must not draw any current is it must essentially have very large resistance.For converting a galvanometer into a voltmeter of a desired range an appropriate high resistance of the order of kilo ohms is connected is series with it. This resistance is commonly known as multiplier resistance. Consider a galvanometer G whose resistance is Rg and which deflects full scale for the current I. Suppose we want to convert it into a voltmeter measuring a P.d upto V volts. An appropriate high resistance Rx must be connected in series with such that for the P.d V applied between the ends of the above combination the current in the galvanometer is Ig which produces full scale deflection. Now the total resistance between the terminals is Rx+Rg thus by the ohms laws. (Rx+Rg)Ig = V Rx+Rg = V/Ig This equation helps to calculate the value of the series high resistance fo the conversion of the galvanometer into a voltmeter of any desired range V volts. When the proper high resistance is connected in series with the galvanometer it is converted into a voltmeter of range V volts the scale can then be calibrated from 0 to V volts. 