249 
TELEGRAPHIC APPARATUS. 
ratus very sensitive. Let us suppose that the magnetization 
produced by the cores represents a force of 3 ounces, and that 
the opposing spring has a force of 2f ounces. A contrary 
polarity of £ ounce would therefore suffice to liberate the 
armature, which would quit the cores with its whole force of 
2f ounces. And as it is replaced in position by the wheel- 
work, the spring acting upon it may have much tension, since 
that tension has not to be overcome by the current. As soon 
as the armature is raised, the coils are placed in short circuit, 
and the current received passes directly to the earth. Thus 
suppose that 100 parts of current are 
received, if 5 parts suffice to detach 
the armature, the excess passes im¬ 
mediately to the earth without affect¬ 
ing the coils. 
When the armature is replaced in its 
position by the wheel-work, an extra 
current is produced in the same direction 
as the received current. The effect of 
this is the same as if a new current had 
been sent by one of the keys, and when 
the resistance of the line is small a con¬ 
tinuous action of the armature and the 
successive printing of several letters 
might be produced. The formation of 
this current is prevented by interrupt¬ 
ing the circuit of the electro-magnet, 
by means of the cam shaft immediately 
it begins to turn, so that at the instant the armature is put 
back in its place, there is no circuit for the extra current. 
Hughes’ apparatus is much used in Europe, and particularly 
in France, where the instrumental skill of M. Froment has 
brought it to a high state of perfection. It is certainly the 
favourite apparatus of the French telegraphic service. On the 
other hand, in spite of its English origin, it has found no 
favour in England, where indeed it has been almost entirely 
given up. 
On account of its very rapid transmission, this apparatus 
has been adopted on the busiest lines. Its working power is 
great, for it easily sends and prints fifty or sixty messages 
Fig. 17 7. 
