66 
THE TELEGRAPH. 
It is one advantage of the Siemens’ system that carriers may 
be sent from different points in the circuit. 
The valves we have just described are those for the inter¬ 
mediate offices, in which each tube is used for reception in one 
direction and transmission in the other. At the central office 
the end of one tube serves only for the transmission, and that 
of the other for reception. At this office the length of tube on 
each moveable frame may then both be, as shown in Fig. 30, 
arranged for transmission or for reception. At the other 
extreme office a single complete valve is sufficient. 
Messrs. Siemens’ system 
has been used in Great 
Britain onlv from 1870. 
%/ 
In London, the Central 
Office, the City, Charing- 
cross, Temple Bar and the 
Houses of Parliament are 
connected by it. 
The air-current having 
opposite directions in the 
two- tubes traversing each 
intermediate office, the 
arrangement may be com¬ 
pared to that of a railway 
with up and down lines. 
This similarity has caused the telcgra] liic department to adopt 
for the signals required in working, Mr. Tyer’s electrical 
apparatus, which is used on a great many railways in the 
United Kingdom, for safety in running the trains. The 
complete apparatus at the Central Office is connected with 
only one office, and consists of an electric bell, a case contain¬ 
ing the electro-magnets, the two handles controlling the 
needles, and another handle for ringing the correspondent’s 
electric bell. The upper needle is painted black, and the lower 
one red. A single line wire is sufficient lor the transmission 
of the various signals. The black needle can be moved only 
b}' the correspondent ; it marks signals received ; the red 
needle repeats the signals sent, so that the operator himself 
sees them. 
The armature of the electro-magnets is a piece of steel, or of 
