256 
THE TELEGRAPH. 
that of the real line, and thus the vibrations caused by the dis¬ 
charge of the real line are counteracted by those of the artificial 
line. The latter must therefore compensate at once the statical 
and the dynamical effects of the currents. 
In submarine cables the induction effects are somewhat 
complicated, and the jerks caused by the charges were, for a 
long time, an obstacle to the application of the duplex system 
on great lines. In addition to the induction effects, there 
occur with these cables certain retardations and modifications 
of the current that could not easily be reproduced in the 
artificial lines. A combination of condensers and rheostats, 
uniformly distributed, proves insufficient for submarine lines of 
great length. In fact, the current must not only traverse the 
artificial line in the same way that it passes through the real 
line, but it must also keep the same form and intensity in both 
circuits, or, in other words, travel from end to end of the two 
circuits like a single undulation. The difficulty of balancing a 
cable and an artificial line increases more rapidly than in pro¬ 
portion to the length of the former. The importance of these 
phenomena was perceived as early as 1855 by M. de Sauty, 
who then used a condenser to compensate the induction effects 
in experiments carried on between London and Birmingham 
by the differential method of Siemens. When, in 1868, 
Stearns took up the subject afresh, he experienced the same 
difficulties, and on the long American land-lines he surmounted 
them only by combining condensers with resistances. Yet on 
land lines, the apparatus does not work in conformity with the 
capacities of the lines ; whilst, with cables, Sir W. Thomson’s 
delicate apparatus, the mirror galvanometer and the siphon 
recorder that are so generally used, respond to much feebler 
currents than those required to form the signals in land lines. 
The balancing of the duplex apparatus does not require to be 
so delicate for land lines as for cables. 
Experiments made by Dr. Alexander Muirhead and Mr. 
Herbert have shown that currents enter the cables by a certain 
kind of undulation, as it w T ere, and it is necessary to imitate in 
the artificial line all the phenomena of the current from the begin¬ 
ning. Guided by these observations, and embodying the 
principles arrived at by his brother, John Muirhead obtained a 
patent in October, 1874, for the artificial line that forms the 
