This a somewhat arbitrary division.
Unless specifically known, under “Maritime keys” I have listed those manufactured by well known suppliers of such equipment or those found on board of ships going to the breakers´ yard (a few cases).
In other cases “ Spark keys” are simply defined as such by the diameter of the operating contacts.
The first transmitters were “Spark Transmitters” intended for maritime use with the sending keys handling heavy currents, thence the need for heavy contacts for them. Later transmitters, with much higher power, also required the use of heavy contacts in the sending keys.

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