Resistance of an Ideal ammeter is:

Explanation:

Ammeter

• An Ammeter is a device which is connected in series of the circuit to find the current. 
• Ampere is the unit of current. 
• Ammeter is connected in series. So, if it is having a high resistance, it will affect the reading as the current will be reduced in the circuit. 
• So, an ammeter should have very small (ideally zero) resistance so that perfect reading can be obtained. 

​Important Points

• For the voltmeter, the resistance should be infinite. 
• If resistance is low, it will provide an easy path for current and overall current will increase. This will affect the readings. 

 Additional InformationConverting Galvanometer to Ammeter

• A galvanometer can be converted into an ammeter by connecting a shunt resistance in parallel to it.
• The shunt resistance should have very low resistance. So, the ammeter (the parallel combination of galvanometer and shunt resistance) will have low resistance.
• To convert a galvanometer into an ammeter of current rating ‘I’, a small resistance ‘S’ (shunt resistance) is connected in parallel across the galvanometer.

Converting Galvanometer to Voltmeter

• A galvanometer may be converted into a voltmeter by connecting a large resistance R in series with the galvanometer.