This is not an answer to any of the above, but i thought it may be of interest. I have in my posesion, a 1920,s dimmer switch. It is a large 'jelly mould' shape in brass with brown ceramic mount. At first glance it looks like a standard togle type light switch exept for the size - diam 95mm, depth 90mm - and the fact that it has many ventilation holes in the brass cover. The toggle however slides up and down. This is connected to two blades which in turn slide against two sets of carbon pads. Iy was new and boxed when I found it while clearing out an old store and to my knowledge has never been used. It is stamped with a rating of 70 watts & 110 volts DC and after a bit of digging around I have dicovered that very early electricity was generated localy and supplied as 110volts dc to a few of the better properties in our town. I think that the supply was intented for very early domestic lighting only.
Does anyone know anything else/conflicting etc