edgeways, appear serrated, or up and down, like a section of
country or the trace of the stylus in the record of a phonograph.
The gelatine plate thus carved by the action of light and water is
wrapped round a revolving drum or barrel, and a spring stylus or
point is caused to pass over it as the barrel revolves, after the
manner of a phonographic cylinder. In doing so the stylus rises
and falls over the projections in the plate and works a lever
against a set of telegraph keys, which open electric contacts and
break the connections of an electric battery which is joined
between the keys and the earth. There are four keys, and when they
are untouched the current splits up through four by-paths or
bobbins of wire before it enters the line wire and passes to the
distant station. When any of the keys are touched, however, the
corresponding by-path or bobbin is cut out of circuit. The
suppression of a by-path or channel for the current has the effect
of adding to the "resistance" of the line, and therefore of
diminishing the strength of the current. When all the keys are
untouched the resistance is least and the current strongest. On
the other hand, when all the keys but the last are touched, the
resistance is greatest and the current weakest. By this device it
is easy to see that as the stylus or tracer sinks into a hollow of
the gelatine, or rises over a height, the current in the line
becomes stronger or weaker. At the distant station the current
passes through a solenoid or hollow coil of wire connected to the
earth and magnetises it, so as to pull the soft iron plug or
"core" with greater or less force into its hollow interior. The up
and down movement of the plug actuates a graving stylus or point
through a lever, and engraves a copy of the original gelatine
trace on the surface of a wax or gelatine plate overlying another
barrel or drum, which revolves at a rate corresponding to that of