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POST OFFICE POLARISED SOUNDER, 1890's
Receiving instrument for Morse code, invented by CC Vyle in the late 1800's and in use until the 1960's.
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A bar (armature) is moved between two stops, pulled down by a permanent magnet positioned underneath the cores of two coils; offsetting this force is an adjustable spring pulling the armature upwards. If the coils are energised by a positive current it assists the magnet and pulls the armature down. A reverse current defeats the magnetic force and allows the armature to rise. The same principal is used in polarised relays. The device can be adjusted such that the tension will allow the bar to stick on the bottom stop until a reverse current is applied. Adjusting the spring can also vary the current needed to operate the armature, this is useful in certain applications. The sounder's advantage lies in its sensitivity, requiring very low current to move the bar (armature), normally a positive current would give a dot or a dash and a negative current a space. Non Polarised Sounders do not have a magnet.