Which instrument is used to measure the potential difference? Also, mention the unit of potential difference.

The device that is used to measure potential difference is a voltmeter. It is an instrument for measuring the electric potential difference between two points in an electric circuit. 

Hence, the potential difference between two points is said to be 1 volt if work is done to transfer 1 coulomb of charge from one point to the other in one joule. 

The S.I. unit of potential difference is joule per coulomb; it is named volt (𝑉). 

It can define as 

We know that potential difference (𝑉) is given by:

$\mathrm{V}=\frac{\mathrm{w}}{\mathrm{q}}$

$\text{ Unit of potential difference }=\frac{\text{ Unit of work }}{\text{ Unit of charge }}$

$1\mathrm{V}=\frac{1\mathrm{J}}{1\mathrm{C}}$