Help me friends,

I'm writing an Historic Structure Report, and am trying to date the various
electrification campaigns.  Do you have any leads for where I can get
information about early electrical switches (i.e. dating them)?  Or patent
information for light switches?  I've got the knob and tube info, but can't
seem to find anything about switches.  We also have surface applied wood
tracks in which the electrical wiring runs.  I've never seen that before
either.

Thanks from Chicago,
Anne the Architect