TELEGRAPHIC APPARATUS. 
221 
which would be contained in three nautical miles of submarine 
cable. 
As the signals given by the mirror galvanometer leave no 
trace, the instrument has the defect of all apparatus that give 
but temporary indications, and if therefore it is employed with 
long cables in preference to any other, it is because it is almost 
the only instrument that will work on these long circuits 
without disturbance. 
Sir W. Thomson’s siphon recorder however is used with the 
long cables of the Eastern Telegraph Co. It may be hoped there¬ 
fore that the same instrument may some day come into use 
with the Atlantic cables. In this apparatus, the chief difficulty 
to overcome, was to obtain distinct marks from a very light 
body put into rapid movement. This result was obtained by 
means of a capillary glass-tube, from the end of which a solu¬ 
tion of blue aniline is discharged in a very slender stream, by 
being electrified statically. A little machine, which has been 
called the mouse mill, produces the supply of electricity for this 
purpose. The siphon is moved by a small coil of fine wire, 
placed in an intense magnetic field. Powerful electro-magnets 
constantly traversed by the current from a large battery, mag¬ 
netize this field. The coil of fine wire moves freely between 
the poles of the electro-magnet, and round a fixed core of soft 
iron. It communicates its movements to the siphon by means 
of silk fibres suitably arranged. 
Sir W. Thomson makes his apparatus more effectual and 
more easy to regulate by using graduated derivations (or 
shunts) by which the motions of the coil may be reduced. 
This coil, with its soft iron centre, and the connections with 
the siphon, &c., forms a system fixed to an insulated freely 
moveable piece, that can easily be adjusted when necessary. 
The coil can be regulated in its movements by the torsion of 
wires, so that a longer or shorter range of motion can be 
obtained (fig. 158). 
The siphon shape would no doubt allow of the flow of liquid 
to be obtained by simple atmospheric pressure, but in the 
apparatus we are describing, the ink is expelled by the repulsion 
of statical electricity produced by a small induction machine, 
worked by an electro-magnetic engine of novel construction. 
The induction apparatus is so made that only the accumu- 
