For another example of how differing filament resistances in bulbs can be made to work in "unusual" ways in a bridge arrangement, just look at how the front side marker lights are wired on General Motors cars.
Instead of just connecting betweeen the parking/tail light circuit and ground as with most Ford and Chrysler vehicles, they connect the marker between the parking light and the appropriate turn signal feed.
When a turn is signaled, the 5W or less side marker bulb is in series with the parallel combination of all the parking/tail lights, so it flashes with the turn signal while the other running lights don't get enough power to light.
With parking/running lights turned on, the side marker is in series with the front turn signal lamp. Again, the small side marker lamp has a much higher filament resistance compared to the much larger turn signal lamp, so the marker lights. (*)
When both running lights and turn signal are turned on, the marker lights via the front turn signal lamp during the off periods of the flasher, then goes out during the on periods as both sides are at 12V, hence the way that the marker flashes alternately with the main signals when the running lights are on.
(*) For a demo of this, try removing a front parking/turn signal bulb and then switching on your parking/tail lights. The green warning light on the dash is connected in parallel with the front turn signal, so with the main bulb removed the side marker is then in series with just the dashboard warning lamp, which is of similar rating to the side marker. You should see the arrow on the instrument panel glow dimly, and the side marker itself will be dimmer than usual too. (If you have one of those cars with independent amber turn signals on the rear, you might need to remove that bulb too, depending upon how it's wired.)
[This message has been edited by pauluk (edited 07-16-2006).]
