Suppose you are using a multimeter (one designed to measure a range of voltages, currents, and resistances) to measure current in a circuit and you inadvertently leave it in a voltmeter mode. What effect will the meter have on the circuit? What would happen if you were measuring voltage but accidentally put the meter in the ammeter mode?
If multimeter is in the voltmeter mode, its internal resistance R will be very high. Assume the resistance of circuit is RC. When this voltmeter is used in the circuit its equivalent resistance becomes very high.
Rc = Rc + R
When this set-up is used to measure the current in the circuit, it will restrict the current flow to a high extent and thus the circuit will become current deficit and the correct reading for current could not be made.
If multimeter is in the ammeter mode, its internal resistance R will be very low.
If this setup is connected in parallel to the circuit, in order to measure the voltage in a segment of the circuit, because of low value equivalent resistor of the ammeter, a large amount of current is extracted by the multimeter, creating a high order voltage drop in the circuit beyond this point.
And hence reading of the voltage in the circuit is incorrect.