364 Recent Advances in Telegraphy. [july^ 
long submarine cables. Nevertheless, by the help of the 
devices above mentioned, it is probable that cables of a con- 
siderable length might be duplexed with an artificial line of 
this Kind. As yet, however, this has not been accomplished, 
except experimentally. 
What is now known as Muirhead’s system is really the 
first successful method of working submarine cables on the 
duplex principle. Mr. John Muirhead, jun., first patented 
this system in 1874, an d since then it has been rapidly de- 
veloped and applied, chiefly by Dr. Muirhead and Mr. 
Herbert Taylor, C.E. The system includes both a novel 
artificial line, or model cable, and various adjustments or 
devices for bringing the artificial line into close equivalence 
with the cable, and for perfecting the balance. Muirhead’s 
artificial line aims at complete equality with the cable, as 
far as it can practically be done. Like a real cable, it is an 
intimate union of resistance and capacity, and is called an 
“ inductive resistance.” It is formed “ by taking two strips 
of tin-foil, and laying one over the other separated by an 
insulator, such as paraffined paper. One strip forms the 
conducting circuit of the artificial line ; the other forms the 
outer or inductive coating, and is connected to earth. The 
current is passed through the conducting strip, and exposed 
Muirhead’s Artificial Line. 
CcmxT/A/tirtg Strip 
toJZar&u 
Jnxtzictcrug 
tFlrcte- 
Tig, 
8 
i 
r 
throughout its entire length to the induction of the other 
strip or sheet in the same way that the current in the con- 
ductor of a cable is subjected to the induction of the earth 
“ throughout its entire length.” This is shown in Fig. 8, 
where the arrows represent the local current passing along 
the conducting strip, while being at the same time retarded 
by the induction of the other strip, or inducting-plate, which 
is connected to earth. This model cable can be made to 
have the same resistance, capacity, and even leakage, per 
knot that the actual cable has, so that it may be practically 
equivalent to the actual cable. For very long lines, how- 
ever, and with such delicate receiving instruments as Sir 
William Thomson’s “mirror galvanometer ” and “siphon 
recorder,” it is in general necessary to refine upon the 
balance by means of extraneous adjustments. It may be 
