एक पदार्थ जो बहुत कम तापमान पर शून्य विद्युत प्रतिरोध प्रदर्शित करता है, उसे किस रूप में जाना जाता है?

The potential difference (V) between two points is defined as:

Which instrument is used to measure the potential difference between two points in an electric circuit?

Which law states that V = IR?