PNEUMATIC TELEGRAPHY . 
67 
iron polarized by a magnet; the outer needle with which it is 
connected inclines to the right or left according as a positive 
or a negative current is sent, by touching one or the other of 
the handles. After each passage of a current, the needle 
regains its position by reason of the residuary magnetism, 
which is made as powerful as possible by the way in which the 
apparatus is constructed. This magnetism is sufficiently 
powerful to bring the needle to its normal position, if it be 
withdrawn by the hand from that position when the current 
has ceased to pass. The magnet that polarizes the armature 
is re-magnetized each time the current passes. 
Fig. 30 . 
The advantages of this apparatus consist in the absence of 
adjustments, in the readiness with which tie needles resume 
their normal position, in the preservation of the magnetism 
notwithstanding discharges of atmospheric electricity, and in 
the adherence of the needles opposite the poles of the electro¬ 
magnet, so that their indications are not affected by vibrations 
caused by the passage of trains. We may, however, point out 
that a single accidental current, such as would be occasioned 
by the approach of a thunder-cloud, or the touch of a telegraph 
wire, might cause the needles to assume a false position. 
Every time a current is sent, the hammer of the electric bell 
strikes once. 
The plan adopted in the despatch of carriers is that known as 
