TVS diode for use as a snubber across the sounder terminals.
Two types are available:
a) for instruments of 100 Ohms or lessb) for instruments greater than 100 Ohms
The diodes are selected assuming that the loop current running through the instrument is within the recommendations found on the MTC website for Electrical Specifications for Telegraph Instruments. A photo of a non-destructive type of installation on a sounder can be seen at http://kob.sdf.org/morsekob/interface.htm#snubbers (solder it to the ring terminals to ensure a good connection).
Additional TVS Diodes can be ordered with no charge for shipping if included with an interface order.