Publication: The Mechanics Magazine
CONTRACTING OF ELECTRIC WIRES—IMPROVED INSULATORS.
Sir,—In your Magazine of last week, No. 1377, Mr. Gilbert, in answer to my letter, suggests, that " if each wire had a kind of spiral twist, at proper distances apart, it would expand or give way sufficiently to counteract the contraction of the metal." Mr. Gilbert is probably not aware that the wires wind up only twice in every mile, and that the wire (No. 8, not No. 1, as printed in No. 1876) being one-sixth of an inch only in diameter, the twist would not bear the strain on it, about three hundred weight.
In my former letter I mentioned that a new kind of bell insulator had lately been invented. I now send you a rough sketch of it. The scale is about one-third of the natural size.
Figs. 1 and 2 and the two sections figs. 1a 2a will, I think, be sufficient to show the principle of this insulator.
The shaft E C is not made round, as in Mr. Rlcardo's bell cone (fig. 3), but of the shape A B, shown by the section at c d; the space C at the end of it is sufficiently large to enable the top of the hook (which top is made of the same shape at the section of the shaft) to turn round in it; the notch D C (section a b) is at right angles to the shaft, and of the same shape, but only about three-sixteenths of an inch deep; so that when the hook has been pushed up the shaft, it is only necessary to give the insulator a quarter of a. turn, and the top of the hook will lock itself into the notch D C, and be kept there by the weight of the wire.
Fig. 3 shows the insulator of Mr. Ricardo's patent, which I described in my former letter; the hollow D is that which has to be filled up with cement or mastic when the hook has been screwed up.
Fig. 5 shows the hook of the improved bell cone.
I find that the first idea of the bell insulator is due to Mr. Alexander Bain, to whom electro-telegraphy owes so many valuable inventions. He used, about five years ago, a bell-shaped insulator, of which I believe fig. 4 to be a section; in this the wire was not suspended from the earthenware by means of a hook, but was looped round the top B, and the insulator itself was supported on a piece of wood or iron which filled the hollow A.
I am, Sir, yours, &c.,
THEODORE G. A. CHESNEL.
Hempstead, January 1, 1850.