Liquid Dimmers (1915 - 1945)
Strand

Glazed earthenware jars, complete with fixed & moving lead cones.
(Made by a number of companies, not just Strand)

Also known as salt water dimmers and electrolytic dimmers.
Liquid dimmers can be used on DC or AC supplies.
The concentration of electrolytes in the water can be changed to enable the dimmer to be used on a range of diffferent loads. The salts are provided by washing soda crystals.

The last electrolytic dimmers were replaced in London in 1959.