Polarised British Telegraph Sounder

British polarised telegraph sounder, receiving morse code transmission by listening to the sounder’s ‘clicks’.
The armature moves between two stops; a permanent magnet pulls the armature and an adjustable spring applies offsetting force; the armature stays in place until opposing current moves it to the other stop.
The current in coil D sets the armature to the downward stop; the current in coil U sets the armature to the upward stop; polarised current when the two coils are in series moves the armature up and down.
The technological advantage of the polarised technology is high sensitivity, requiring low current to move the armature.
Used by the British Post Office and the British Military since 1911.
Used during the British Mandate.