We can detect weak current flowing through a liquid by two methods
Using LED
LED is a device which glows even when very small amount of current is passed through it.
Hence, it can be used to detect weak currents as it will glow even when very small current is passed through it.
Compass surrounded by wire
A magnetic field is created around a current carrying wire even when very small amount of current is passed through it.
A compass needle shows deflection in the presence of magnetic field.
Hence, a compass can be used to detect a weak current.
