TELEGRAPHIC APPARATUS. 
257 
base of their system of duplex telegraphy as applied to sub¬ 
marine cables. Since that period the system has been 
developed and applied by Messrs. Muirhead & Taylor. All the 
cables of the Eastern Telegraph Co. between England, Mar¬ 
seilles, and Bombay have been duplexed on this system. The 
same has been done with the United States Direct cable. The 
first cable that was thus duplexed was that between Marseilles 
and Bona in 1875 ; and the system has lately been applied to 
the Vigo cable. 
The system requires a new artificial line, a model of the 
cable itself; then various adjustments to give the commence¬ 
ment of the artificial line the greatest possible resemblance to 
the commencement of the cable. This was accomplished in a 
Fig. 181 . 
general manner by giving the artificial line the form of a con¬ 
tinuous inducer of uniform capacity throughout its whole 
length. This may be called the “ inductive resistance.” It is 
produced by means of two strips of tin-foil separated from each 
other by paper charged with paraffin. One of these strips con¬ 
stitutes the conducting circuit of the artificial line ; the other 
is the external inductive part, and communicates with the 
earth. This arrangement is shown in fig. 181, representing a 
part of the artificial cable. The conducting strip is con¬ 
tinuous ; but the binding screws, n, are fixed at intervals, and 
resistances may be thrown in. The strips, or rather groups of 
inductive strips, are, it must be remembered, separated from 
each other ; but each group is connected with its own binding- 
screw, and these are all connected with the earth. Each pair 
of screws, n, combined with the corresponding capacity c 
opposite, thus forms an element of a series each member of 
which is equivalent to about ten nautical miles of the real 
cable. This method of construction gives us an artificial cable 
having the same resistance per unit of capacity as the real 
cable that has to be duplexed. 
