why is there a small potential found in the electronic multimeter in a completely connected circuit even when the input potential is kept zero(using rheostat)?

Question

anonymous · Answer

A basic voltmeter circuit consists of a resistor in series with a measuring device.  Typically, the resistor has a large value so that it looks like an open circuit.  With a large input impedance to the voltmeter, it doesn't affect the circuit-under-test.  
     Resistors exhibit Johnson noise (aka thermal noise).  The formula for the amount of noise present is:
$$V=\sqrt{4*k*T*R*\Delta f}$$
'k' is Boltzmann's constant (joules), 'T' is absolute temperature (Kelvins), 'R' is the resistor value (Ohms), and delta f is the bandwidth (Hz).  Noise is unavoidable unless your at absolute zero!  
     This discussion only covers the noise from the resistor in the voltmeter/multimeter.  Noise can be added from the measuring device in the voltmeter, the power supply, other portions of the circuit, etc...