Listening to the noise made by a sounder and determining the message being sent
required incredible skill. It also required an operator to be present at all times in order to
receive every message.
Pen registers or ‘inkers’ were machines that could print the Morse Code dots and dashes directly onto paper tape.
The incoming Morse signals pass through electromagnets which, just like a sounder, attract an
armature which is connected to a moving inker arm. When the electromagnet attracts the armature so the
inker arm moves upwards, pressing the paper tape against an inked roller. In this way a dot will generate a
short ink mark and a dash a longer ink mark on the paper tape. The paper tape is wound on a reel and is
driven by a clockwork mechanism and a set of feed rollers.
However, the really interesting part about this machine is its automatic start and stop mechanism.
In other words how does it know when a message begins and ends?
Picture left shows the insides of the pen register. When the first dot or dash of the message is received,
the armature of the electromagnet is attracted and is connected via a linkage to a moveable pin.
In its normal position, this pin interrupts the clockwork mechanism which is held in a stop position.
However, when the armature moves, so the pin moves and releases the clockwork mechanism.
A spring pulls the pin to engage it with a timing thread. As the clockwork mechanism runs, so this timing
thread rotates and moves the pin back towards the clockwork mechanism stop. However, each successive
dot or dash continues to move the pin away from the clockwork stop. Therefore whilst the message is being received,
the pin is kept away from the stop and so the clockwork mechanism continues to run and feed the paper tape off the reel,
through the feed rollers and between the inked roller and inker arm. After the last dot or dash of the message is received
so the pin remains engaged in the timing thread and moves back towards the clockwork mechanism stop.
When it reaches the stop, the clockwork mechanism is stopped. Adjustments are provided to control where on the
timing thread the pin is moved to by the armature.
The further along the thread, so the longer it takes for the pin to return to the clockwork stop.