248 
THE TELEGRAPH,\ 
and type wheel are properly adjusted, a current is sent through 
the line by a touch of the key at the very moment when the 
corresponding letter is in front of the paper, and the paper 
being at the same time raised up to that letter by a movement 
produced by the current, receives its impression. 
There is an arrangement for regulating the speed of rota¬ 
tion, and the errors that may be caused by a difference of 
speed in sending and receiving apparatus are rectified by a 
wheel called the “ correeting-wheel.” 
The current prints the letters in this way:—A hollow shaft 
carrying three cams is fitted on the axle of a wheel that is 
geared with the axle of the type-wheel. This hollow shaft is 
not carried round by the axle that passes through it, except 
when the two are joined by a click brought into action by the 
current. Thereupon the axle and hollow shaft turn round 
together, and the following results are produced by the opera¬ 
tion of the cams :— 
1°. The first cam corrects the position of the type-wheel. 
2°. The second cam presses the paper against the type. 
3°. The third cam carries the paper forward. 
The connection between the hollow shaft and the axle is 
then broken, and the shaft stops. A fourth cam replaces the 
armature on the electro-magnet, and the apparatus is then 
ready to receive the next signal. 
The electric action, although simple, presents some pecu¬ 
liarities. The electro-magnet has two soft iron cores covered 
with fine wire, and each of these cores rests on a pole of a per¬ 
manent magnet (fig. 177). The armature is also made of soft 
iron, and when it is placed on the electro-magnet, it is attracted 
and retained by the polarity induced in the cores by magnetism. 
A spring adjustable by means of a screw tends constantly to 
free the armature. If a current traverse the coils in such a 
direction as to produce a polarity opposite to that induced by 
the permanent magnet, the armature will be separated, and as 
soon as that happens it will leave the cores abruptly with 
sufficient force to liberate a detent. The armature is after¬ 
wards brought back to its place by the action of the wheels. 
The attraction between the cores and the armature is regu¬ 
lated by a small bar of soft iron placed between the poles of 
the permanent magnet. This arrangement makes the appa- 
