246 
THE TELEGRAPH. 
a variable resistance apparatus to prevent false currents from 
entering the receiver. 
Automatic apparatus are used on nearly all the great 
European lines. It is evident that the principal advantage of 
an automatic system consists, not only in its great exactness, 
but also in the increased speed of transmission. The average 
speed may reach seventy words per minute, that is to say, a 
wire will convey twice as many words with a Wheatstone as 
with a Morse transmitter. But the former is more expensive 
in working, for when it is in constant use, two clerks are 
required to work the perforators, another to adjust the 
apparatus, and there must be three writers at each end. 
The despatches are perforated and sent in sets of five, and 
each must therefore wait for the perforation, again for its turn 
in the set, and also its transcription at the receiving station. 
These are all so many causes of delay that there is no 
economy in employing the automatic system on short lines. 
It is therefore used only on long important lines. 
Hughes' printing telegraph .—This apparatus differs from 
all others in this respect, that the action is principally pro¬ 
duced by machinery, the electric part being confined to the 
transmission of a brief current when types set on a wheel are 
in the required position. A single electric impulse then 
suffices to print a letter in ordinary type. The receiving and 
transmitting apparatus are identical, and they are manipulated 
on a key-board provided with as many keys as there are letters 
or signs to be printed. Arranged in a circle are a set of pins 
each connected with one of the keys ; in the centre of the pins 
revolves an axle carrying on a projecting arm the “ chariot ” 
which sweeps round the circle formed by the pins, without, 
however, touching them, so long as they remain in their 
normal position. But when one of the keys is pressed, the 
corresponding pin is raised; and the “ chariot/’ by touching it 
in passing, is caused to make a contact which sends into the 
line a current that passes through the electro-magnets of the 
sending and the receiving instruments. 
The type-wheel is carried round on an axle engaged with 
that which carries the “ chariot.” But the type-wheel is so 
arranged that it can, when required, be stopped or adjusted 
without suspending the general movement, WTten the “chariot’* 
