How can the potential difference be measured?

Potential difference refers to the difference in potential across a nerve cell membrane. This can be measured using a small-tipped microelectrode with a tip that measures about a micron. The extremely small tip allows the microelectrode to penetrate the cell without damaging it.  

No potential is recorded when the electrode is in the extracellular medium or the bath. This is because the extracellular medium is isopotential. However, on inserting the microelectrode carefully into the cell, a dramatic change in potential is recorded, with the voltmeter reading instantly changing from 0 mV to a potential difference of -60 mV inside the cell relative to the outside. This potential, recorded when a living cell is pierced with a microelectrode, is known as the resting potential. The resting potential varies from one cell to another.

The resting potential usually remains constant in the absence of any stimulation.