Darren Logan wrote:

>  When water  droplets landed on the tile, the water acted to bridge
> the tracks and hence lower the resistance between the tracks.
> However, after the initial application of a water droplet, the
> resistance would quite rapidly increase (over minutes not hours).

  Remember that rainwater in a clean atmosphere is an insulator.

>  I'm no scientist, but I guess this was something to do with the
> current passing through the droplet - doing 'something' to the
> conductivity of the water.

  If it was rainwater, the only reason the droplet would conduct in the
first place would be contamination - of either the rainwater (or other
condensate) or the surface.  Very small amounts of contaminant would be
dissociated by the current and react with the plates, so that the
electrolyte would be removed.

>  Or maybe there was some sort of chemical build up of insulation
> around the base of the droplet... ??

  As someone else mentioned, "polarisation" may occur as gas bubbles
(oxygen or hydrogen) coat the surface.
--
  Cheers,
        Paul B.