You might want to measure the current consumed by your Arduino.
Usually one uses a shunt resistor (e.g. placed in the negative voltage feed) and measures the voltage at this shunt.
The problem is that -seen from the Arduino- the voltage at the shunt is negative and the ADC can’t cope with negative potentials.
So we will use a dirty trick: plain silicon diodes begin to conduct @ 0,5 - 0,6 V.
Using that circuit:
you will roughly add 0,5 - 0,6V to the potentials around the shunt and shift them into positive values that can be read by the ADC.
The current will then be roughly proportional to A0-A1.
You must chose the shunt resistor value to generate a maximum tension of ~0,2V to get a reasonable precision and use matched diodes, or at least of the same type.