In the process of construction, the central wire of the copper core was first covered with a semi-liquid coating of gutta-percha, mixed with tar--known as "Chatterton's Compound." This was laid on so thick that when the other wires were wound round it all air was excluded. Then a coating of the same compound was laid over the finished conductor, and thus the core was solidified. Next, the core was surrounded with a coating of the purest gutta-percha--a splendid non-conductor, impervious to water--which, when pressed to it, while in a plastic state, formed the first insulator or tube to the core. Over this tube was laid a thin coat of Chatterton's Compound for the purpose of closing up any small flaws or minute holes that might have escaped detection. Then came a second coating of gutta-per
