210 
THE TELEGRAPH. 
needles with their poles reversed in position, the apparatus 
becomes still more delicate; but the most sensitive apparatus 
of the kind is Sir William Thomson’s mirror galvanometer, 
which will be described in the next chapter. 
Advantage has been taken of the action of currents on the 
magnetic needle in the formation of telegraphic apparatus in 
which the signals are produced by the deflection of the needle 
to the right or left, according to the direction given to the 
current. 
As the action of currents and magnets on solenoids is like 
that on magnetic needles, some forms of apparatus are based 
upon this fact, as for example, Sir W. Thomson’s siphon 
recorder. 
Very powerful mechanical effects can also be obtained by 
electro-magnets. Arago was the first to observe that under 
the influence of an electric current, pieces of soft iron in the 
vicinity are converted into magnets. If a bar of soft iron be 
placed in a coil, it is magnetized whilst the current is passing, 
but the magnetism disappears the moment the current is inter¬ 
rupted. The coil is made of insulated copper-wire, wound 
spirally a great number of turns round the bar of soft iron. 
In practice the soft iron bar is bent into a horse-shoe or 
other like form. When the current stops, the electro-magnet 
reverts at once to a neutral condition, if the iron is pure and 
has been properly annealed. 
The power of electro-magnets depends upon :—1st, the 
intensity of the current; 2nd, the number of turns made by 
the wire in the coil; 3rd, the nature of the iron and its shape ; 
4th, the shape and size of the armature. In general, it may 
be said that, other things being equal, the electro-magnet is 
more powerful as the diameter of the iron is greater; that the 
magnetic intensity is proportional to the current, and to the 
number of turns made by the wire. 
The facts stated above will, it is hoped, suffice to make our 
readers understand the action of the apparatus to be described 
in the next chapter. We have not considered it desirable to 
enter into details on the theory of electro-magnetism, but those 
who desire to pursue this matter may consult some of the 
already published treatises on electro-magnetism. 
