transmitting and receiving drums are respectively connected to earth.

In operation the perforated tape is placed on the transmission drum, and
the chemically prepared tape on the receiving drum. As the perforated
tape passes over the transmission drum the advanced rollers 6 or 8
first close the circuit through the perforations, and a positive current
passes from the batteries through the drum and down to the ground;
thence through the earth at the receiving end up to the other drum and
back to the batteries via the tellurium pins 2 or 4 and the line wire.
With this positive current the tellurium pins make marks upon the
paper tape, but the iron pins make no mark. In the merest fraction of a
second, as the perforated paper continues to pass over the transmission
drum, the rollers 5 or 7 close the circuit through other perforations
and t e current passes in the opposite direction, over the line wire,
through pins 1 or 3, and returns through the earth. In this case the
iron pins mark the paper tape, but the tellurium pins make no mark. It
will be obvious, therefore, that as the rollers are set so as to allow
of currents of opposite polarity to be alternately and rapidly sent
by means of the perforations, the marks upon the tape at the receiving
station will occupy their proper relative positions, and the aggregate
result will be letters corresponding to those perforated in the
transmission tape.

Edison subsequently made still further improvements in this direction,
by which he reduced the number of conducting wires to one, but the
principles involved were analogous to the one just described.

This Roman letter system was in use for several years on lines between
New York, Philadelphia, and Washington, and was so efficient that a
speed of three thousand words a minute was attained on the line between
the two first-named cities.

Inasmuch as there were several proposed systems of rapid automatic
telegraphy in existence at the time Edison entered the field, but none
of them in practical commercial use, it becomes a matter of interest to
inquire wherein they were deficient, and what constituted the elements
of Edison's success.

The chief difficulties in the transmission of Morse characters had been
two in number, the most serious of which was that on the receiving tape
the characters would be prolonged and run into one another, forming a
draggled line and thus rendering the message unintelligible. This arose
from the