Publication: The Mechanics Magazine
ELECTRIC TELEGRAPH INSULATOR.
AT the February meeting of the Franklin Institute at Philadelphia, Dr. Turnbull called the attention of the meeting to the very great importance of proper insulation of the metallic wires of the electro-magnetic telegraph. He exhibited two new forms of insulators. The first was a modification of the form designed by J. M. Batchelder, Esq., of Boston (1), but omitting the use of iron, and being composed of flint, quartz, and feldspar, very compact, thoroughly vitrified on the surface, was equal to the best forms of glass insulators, and much stronger; it is in the form of a cap with a ridge for the purpose of fastening the wire, and an inverted edge so as to divert the rain downward, and prevent it from entering the inside of the cap. He remarked, that even this form of insulator is defective, and the moisture settles upon it, and this acts as a carrier of the electricity to the ground, A still further modification of this apparatus is desirable, so as to give the surface of the insulator a downy covering, to cause the moisture to remain in isolated dress upon it; this, Mr. Batchelder is endeavouring to accomplish. He has also produced a change by heat, &c., in the best electric substance known, namely, caoutehouc, so as to render it impervious to moisture, heat, and rapid decomposition, and fit it for insulating caps for the tops of posts.
The composition is of a dark colour, and in the form exhibited has a ringing noise when struck. Subjected to water at 212° it did not soften; strong sulphuric acid had no action upon it; even pure nitric acid did not destroy its elasticity, while it completely altered a piece of pure caoutehouc, converting it into a mass of brown colour, which, when pressed between the finger, falls to powder. The only change noticed was its colour, which was yellow instead of black. When placed in the flame of gas it burned with freedom, giving off scintillations as if combined with metallic oxide, leaving a polished surface, while ordinary caoutehouc liquid, when burned, produced a pyro-oil which stains the fingers, so that it has all the qualifications of a good insulating substance, being an electric not affected by a heat of 212°, not altered by acids, end not liable to decomposition.
(1) See Mech. Mag, vol. LVI., p. 351.