The Coherer was the first device used to detect
the radio frequency signals of Wireless
Telegraphy. It consists essentially of a small
quantity of metal filings lying loosely between
metallic electrodes. The Marconi Coherer consisted
of a very small quantity of nickel filings,
to which were added a small percentage of silver
filings, lying between silver electrodes having
bevelled ends so that the space between them,
in which were the filings, was wedge-shaped.
The silver electrodes and filings are enclosed
in a glass vacuum tube.
Its operation is based upon the resistance offered
to the passage of electric current by the loose
metal filings. In the presence of radio
frequency energy, the filings cling together (cohere) and
allow a current to pass. This current was used
to actuate a sounder. The filings continued to
cling together after the signal paused, and this
was overcome by tapping the Coherer with a
small hammer (which was attached to the sounder)
after the arrival of each signal.
The Relay/Hammer arrangement was energized by
a battery as shown in the circuit diagram.
The morse code could be read by either listening
to the sound of the relay hammer, or by
connecting a pen to the hammer which printed
dots and dashes onto a strip of paper.
The system could only be used to receive slow
morse, usually less than ten words per minute. |
