Kelvin Bridge: 1862 W. Tang Musun of Britain encountered difficulties in measuring small resistance with a single-arm bridge. He found that the reason for the large measurement error was the lead resistance and the contact resistance at the connection point. These resistance values may be much larger than the measured resistance values. So he proposed a bridge called pometia Sun Qiao. Later, because he was promoted to Lord Kelvin, it was also called Kelvin Bridge.