A voltmeter is basically an ammeter. Every ammeter can withstand a voltage given by the product of its full scale deflection current and its internal resistance. Thus a 0 – 50 micro amperes meter (ammeter) with an internal resistance of 2000 Ω can measure a voltage of 100 mV. In order to obtain the required full scale deflection in volts, a series resistance called the multiplier resistance is to be connected in series to the ammeter. The ammeter and the multiplier resistance in combination will make a voltmeter. The multiplier limits the current to the full scale deflection current of the ammeter when the full scale deflection voltage is impressed across the combination. The arrangement is shown in Fig.

## Ammeter with a series resistance to work as a voltmeter |

**(a) Value of Multiplier Resistance Required to Convert an Ammeter into a Voltmeter:**

_{m}be the full scale deflection current of the given ammeter.

_{m}be the internal resistance of the given ammeter.

_{s}be the value of the multiplier resistance.

_{m}.

_{m}.R

_{m}

_{m}(R

_{m}+ R

_{s})

_{s}= (V/I

_{m}) – R

_{s }--------------------------- 1

_{m}(R

_{m}+R

_{s})/I

_{m}R

_{m}= 1+ (R

_{s}/R

_{m})

_{s}= (N-1)R

_{m}------------------------ 2

_{m}.

**(b) Construction of Multiplier Resistances: **

**Example:**

_{s}

_{m}) - R

^{-3}) - 50

^{2}x 99.95 KΩ

**(c) Multi Range Voltmeters: **

Multi Range Voltmeter |

_{1}, R

_{2}, R

_{3}and R

_{4}will be connected in the 1st, 2nd, 3rd and 4th positions of the rotary switch respectively in series to the ammeter. Thus the four positions of the switch will give four voltage ranges, using the same basic ammeter. In this method of connection we need four individual multiplier resistances to cover the four voltage ranges.

_{1}, V

_{2}, V

_{3}and V

_{4}with increasing voltages respectively. The values of resistors (resistance tapes on single multiplier resistor) are R

_{1}, R

_{2}, R

_{3}and R

_{4}, respectively. The values of these resistors are calculated as follows.

_{m}be the meter current (FSD)

_{m}be the internal resistance of the ammeter.

_{1}= (V

_{1}/l

_{m}) - R

_{m}

_{2}= (V

_{2}/I

_{m}) - (R

_{m}+ R

_{1}) ... as the meter internal resistance along with the first value of multiplier resistance will now be the internal resistance for the second voltage range.

_{3 }= (V

_{3}/I

_{m}) - (R

_{m }+ R

_{1}+ R

_{2}) ... for the same reason as mentioned above.

_{4}= (V

_{4}/I

_{m}) - (R

_{m}+ R

_{1}+ R

_{2}+ R

_{3}) ... for the same reason.

_{1}, which is to be manufactured. The other resistances will be commercially available, if the multiplier resistance is to be made up of four individual resistances. If a taped resistor is used the taps are to be adjusted to the value obtained by calculation.

## 0 on: "Voltmeter | Multi Range Voltmeter"