The cut, Fig. 322, shows a single needle telegraph instrument of the
tapper form. The action of the tapper can be understood from the next
cut.
Fig. 322. SINGLE NEEDLE TELEGRAPH INSTRUMENT, DOUBLE
TAPPER FORM.
Fig. 323. DOUBLE TAPPER KEY FOR SINGLE NEEDLE TELEGRAPH.
C and Z are two strips of metal to which the positive and negative poles
of the battery are respectively connected. E and L are two metallic
springs; E is connected to earth, L is connected to the line; at rest
both press against Z. If L is depressed so as to touch C, the current
from the battery goes to the line by the key L, goes through the coils
of the distant instrument and deflects the needle to one side, and then
goes to the earth. If the key E is depressed, L retaining its normal
position, the direction of the current is reversed, for the other pole
of the battery is connected to the earth and the reverse current going
through the coils of the distant instrument deflects the galvanometer
needle to the other side.
In the drop-handle type an analogous form of commutator worked by a
single handle produces the same effects.