Question

An ammeter can measure current upto 500mA. There are 20 equal division between 0 and 100mA mark on its scale. During an experiment to determine the equivalent resistance of the two resisters joined in parallel a student observes ammeter’s pointer at 3rd graduation mark after zero when the key is off and the pointer at the 17th graduation mark after 200mA when the key is inserted into the plug.The correct value of the current flowing in the circuit is

Answer 1

When the key is off there should be no current in the circuit. 
Our ammeter still measures some current, and this is called hysteresis error. This means that this particular ammeter is going to add some amount of current to any current measured. In order to compensate for this error, we simply we deduct it from all measurements.
Each mark on the ammeter is worth:
$m=\frac{100mA}{20}=5mA$
This means that our error is:
$e_r=3\cdot m=15mA$
When the circuit is closed we measure:
$I_{er}=200mA+17\cdot m=285mA$
This is current with the error, real current in the circuit is:
$I=I_{er}-e_r=285-15=270mA$