Unpublished extract of a letter, 'On the theory of the electric telegraph' from William Thomson to Gabriel George Stokes Thomson aims to indicate some important practical applications of the theory of the electric telegraph, especially in estimating the dimensions of telegraph wires and cables required for long distances. He states that he reserves a more complete development and illustration of the mathematical parts of the investigation for a paper on the conduction of electricity and heat through solids, which he intends to lay before the Royal Society on another occasion. Subject: Electricity Received 3 May 1855. Whilst the Royal Society declined to publish this paper in full, an abstract of the paper was published in volume 7 of the Proceedings of the Royal Society as 'On the theory of the electric telegraph'. 28 October 1854