How did William Thomson explain the delay of electric signals in a long submarine cable?

William Thomson, later known as Lord Kelvin, explained the delay of electric signals in long submarine cables by applying a mathematical analogy between the flow of heat in solid bodies and the flow of electric current in conductors.1

In 1854, when asked for a theoretical explanation regarding the signal delay in a proposed 3,000-mile transatlantic cable, Thomson utilized equations he had previously developed to describe heat flow through a solid wire. He demonstrated that these equations were applicable to determining the velocity of an electric current in a long cable. This theoretical framework allowed him to address the challenges of signal retardation, eventually leading to the adoption of his design recommendations and his invention of the mirror galvanometer to improve telegraphic communication.1