Measuring Instruments

Galvanometer: 

It is used to detect very small current. It has negligible resistance.

Ammeter

It is an instrument used to measure current. It is put in series with the branch in which current is to be measured. An ideal Ammeter has zero resistance. A galvanometer with resistance G and current rating ig can be converted into an ammeter of rating I by connecting a suitable resistance S in parallel to it.

Exercise 9: An ammeter is always connected in series and voltmeter connected to parallel in any circuit element, why?

Illustration 32.A galvanometer having a coil resistance of 100 Ω gives a full scale deflection when a current of 1 mA is passed through it. What is the value of the resistance which can convert this galvanometer into ammeter giving full scale deflection for a current of 10 A?

Voltmeter

It is an instrument to find the potential difference across two points in a circuit.

It is essential that the resistance Rv of a voltmeter be very large compared to the resistance of any circuit element with which the voltmeter is connected. Otherwise, the metre itself becomes an important circuit element and alters the potential difference that is measured.

Rv  >> R

For an ideal voltmeter RV = ∞.

ig(G + R) = V

⇒ R = V/i_g − G

Illustration 33.A galvanometer having a coil resistance of 100 Ω gives a full scale deflection when a current of 1 mA is passed through it. What is the value of the resistance which can convert this galvanometer into a voltmeter giving full scale deflection for a potential difference of 10 V?

Solution: V = Ig [G +Rv]

10 = (10−3) (100 + Rv)