CHAPTEB V. 
TELEGRAPHIC APPARATUS, 
Wheatstone and Cooke—Thomson’s Mirror Galvanometer—Commutators— 
Condensers — Siphon Recorder — Morse Apparatus — Relays — Keys — 
Wheatstone’s Automatic Apparatus—Hughes’ Printing Telegraph— 
Duplex Telegraphy—Earth Plates—Lightning Conductors. 
In the historical sketch we have given of the first attempts 
at telegraphy, it is related how Mr. Cooke, after he had intro¬ 
duced in England a copy of Schilling’s telegraph, transformed 
it—with Wheatstone’s co-operation—into an apparatus, first 
with five needles and five wires, afterwards with only two 
needles and two line wires. Afterwards, Wheatstone reduced 
the number of needles to one, requiring only a single line wire. 
In England this apparatus has remained in use on all the 
railways and some of the least important postal-telegraph 
lines. 
The “ single-needle ” instrument is based on the funda¬ 
mental fact we have explained in the preceding chapter; that 
is to say, that a magnetized needle, placed in the centre of a coil 
of wire will be deflected to the right or to the left according 
to the direction in which the current is sent through the wire. 
There are two forms of the “ single-needle ” apparatus; one 
(fig. 146) with a handle; the other (fig. 148) with two levers 
or keys. The principles of these apparatus are identical, the 
only difference is in the manipulation. Fig. 147 shows the 
apparatus uncovered. A is the receiver. It consists of two 
coils of silk-covered copper wire placed symmetrically on each 
side of the magnetic needle, which is placed in their magnetic 
focus. One end of these coils is in connection with the line, 
the other with the earth. On the same axle as the magnetic 
needle, there is fixed an indicator on the outside of the case. 
The movements of the indicator and needle are limited by two 
