TELEGRAPHIC APPARATUS. 
237 
circuit, and the receiving clerk cannot interrupt the transmit¬ 
ting one, until the latter’s commutator is put back to “ Receive.” 
A galvanometer is always interposed in the line circuit, and 
this instrument has two separate coils, one connected with the 
sending, and the other with the receiving circuit, and this 
enables the transmitting operator to see any interruptions made 
by his correspondent. 
There are many other kinds of key, but we here describe only 
the two forms that are most used in telegraphy. 
Wheatstone's Automatic Apparatus. — Although a good 
operator is able to transmit more than forty words per minute 
with a Morse key, this speed is never continuously maintained 
in practice. The effort cannot long be sustained, and, however 
expert the operator may be, his signals at this speed are not 
always clear and well formed. If a machine that would 
transmit the signals at the highest possible speed were substi¬ 
tuted for the hand, there would be the manifest advantage of 
having clear and perfectly formed signals made by an instru¬ 
ment incapable of fatigue. 
Further, although overhead lines are much less liable to 
induction effects than are submarine or subterranean cables, 
yet with a rapid succession of signals, a similar inconvenience 
is experienced. It is only by using alternate currents at each 
signal, that the speed in overhead lines can be increased. But 
with the Morse alphabet the limit of speed is soon attained, 
since the length of the dots and dashes depends upon the 
duration of the currents. 
A series of dots can be much more rapidly transmitted than 
dots and dashes successively, because the currents sent are 
uniform and charge the line to the same degree ; whereas the 
dash charges it more strongly and therefore requires a longer 
time for discharge, ora longer influx of the contrary current to 
bring it to the neutral state. Besides this unequal effect in 
the duration of the currents, there is also an important action 
on the receiving electro-magnet ; for if a current is prolonged 
the iron is more powerfully magnetized, and therefore requires 
a longer time to revert to the neutral state. In consequence 
of this, the signals made with great rapidity become confused, 
the effect of one current not having ceased before another 
follows ; so that a dot following a dash might simply appear a 
