station these two components are then recomposed by a pantograph
arrangement of taut cords, or levers, and the resultant motion is
communicated to the duplicate pen at that place. The plan adopted by
Mr. Cowper to transmit each continuously varying component is to cause
the resistance of the circuit to vary very closely with the component
in question. Fig. 5 shows how the apparatus is theoretically arranged
for this purpose. P is the writing style, which is held in the
writer's hand in the ordinary way, while he shapes the letters one by
one on paper pulled uniformly underneath by means of clockwork. To P
are attached, at right angles, two arms, a a, one for each circuit;
but as it is only necessary to consider one of the circuits, say that
sending up and down motions, we will confine our attention for the
present to the arm, a. One pole of the sending battery, B, is
connected to the arm, a, the other pole being connected to earth. Now
the arm, a, is fitted with a sliding contact at its free extremity,
and as the pencil, P, is moved in writing, a slides lengthwise across
the edges of a series of thin metal contact plates, C, insulated from
each other by paraffined paper. Between each pair of these plates
there is a resistance coil, C, and the last of these is connected
through the last plate to the line, L. It will be seen that as a
slides outward across the plates the current from the battery has to
pass through fewer coils, since a short-circuits a number of coils
proportional to its motion. But the fewer of these coils in circuit
the stronger will be the current in the line; so that the extent of
the motion of the arm, a, in the direction of its length, that is to
say, the direct component of the motion of the pencil along the line
of the arm, a, is attended by a corresponding change in the current
traversing the line. If the pencil makes a long up and down stroke
there will be a strong current in the line, if a short one there will
be a weak current, and so on. A precisely similar arrangement is used
to transmit the sidelong motion of the pencil along the line, L.

[Illustration: Fig. 5.]

The current from the line, L, flows at the receiving station through a
powerful galvanometer, G, to earth. The galvanometer has a stout
needle, one tip of which is connected to a duplicate pen, P, by a
thread, t, which is kept taut by a second thread stretched by a
spring, s'. The current from the line, L', flows through a simi