GPO NON-POLARISED 20 Ohm SOUNDER, 1930's

Post Office Polarised Sounder

Receiving instrument for Morse code, invented by CC Vyle in the late 1800's and was in use until the 1960's.

A bar (armature) is moved between two stops, pulled down the cores of two coils, offsetting this force is an adjustable spring pulling the armature upwards.
If the coils are energised by a current it pulls the armature down.
Adjusting the spring can also vary the current needed to operate the armature, normally a positive current would give a dot or dash and no current a space.
Non Polarised Sounders do not have a magnet.

Your comments:

• I have one of these (900 Ohm) retrieved from a scrap Test Desk in the Telephone Exchange I worked during the 70s. It was used as a sounder for identifying/proving a circuit on the main frame. If you put a loop on the circuit you were working on (pair of wires) you could hear it from a distance. It is a link back to my Grandfather who worked as a linesman from the same exchange in the 30s. I never met him as he passed away just before I was born.
  .......... David James, Stratford Upon Avon, 28th of April 2020
  
  
• I recently got a 900 ohm version on E-bay in Nov 2011. I built a 30v dc psu and it takes 25m/a to operate it cleanly . The 900 ohms stated is measured at 978 ohms. Looks good, works a treat
  .......... albert heyes g3zhe, warrington, cheshire, uk, 6th of December 2011
  
  
• There are a few different models of this sounder some having low coil resistance 20 ohms ( local sounders )some with as much as 900 ohms ( mainline sounders I have one which has a buzzer built in it can be switched between silent or buzzer.These sounders were made by, Walters electrical, Elliot Brothers, ATM Liverpool and the GPO factories at Silvertown London.
  .......... Roy Frettsome, Mansfield.Nottinghamshire, 18th of January 2010
  
  
• I found one of these in a school laboratory about 15 years ago. It was in pieces and rather corroded, I cleaned it up, re-assembled it and it worked very well. In fact I took it to an amateur radio club, and a number of members were able to read morse at about 15 words a minute, it did require about 50volts to operate, but as typical signalling line voltages were about 70volts this was acceptable.
  
  BTW there is a picture in existence of Marconi using either this model or the polarised version (I suspect the latter) dated 1901.
  .......... Steve Cook, Richmond/Surrey, 21st of December 2009
  
  

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