Old wall switch is mercury switch

We have been replacing the old knob-and-tube wire to bring it up to code. One of the old switches turns out to be a mercury switch.

If you have old wiring (1940s, late 50's?) and an old switch which has no 'click' feel at all, there's a good chance it's one of these. Other types of switches are likely to have worn out years ago, but these will last indefinitely. There shouldn't be a problem with toxicity during use since the mercury is extremely well sealed, but it needs to be disposed of properly (at time of writing I'm not sure how to do that in Toronto).

Here's what it looks like - the screws for attaching the wires face towards the front, which is very unusual, so you can probably identify it that way just by removing the switch plate.
When the switch is removed, you can feel (and hear) the mercury in it just by shaking it in your hand. Picture below shows all the parts - the round thing at the bottom left contains the mercury, sealed between two metal caps separated by an insulator. All that happens when you operate the switch is that the container rotates 90 degrees on its axis, allowing the mercury to connect the two ends.



I found one of these in the last house I owned, too. The only reason it got taken out was that we wanted to replace all the switches with Decora type. It was of exactly the same design (and that house was about the same age).