adding dimmer switch to old wiring

I live in a house built in 1909. Most of the wiring is very old knob and tube. I would like to replace a dining room light switch with a dimmer switch.
Two wires come out of a metal box and screw into the existing switch. They are unmarked. I assume that one is from the power source, and one is the return. The switch just completes the circuit.
My new dimmer switch has a white wire, a black wire, and a green (ground) wire. I believe that the white wire should be connected to the hot wire coming from the wall, and that the black wire should be connected to the return.
But, I have come accross some problems:
1. It doesn't appear that the (metal) box is grounded. I have a simple 120v tester and it does not light up when I touch hot to hot, and ground to the box.
2. Since I don't think I have a ground, I can't tell which of the two wires coming from the box is hot, and which is the return.
3. I am not sure how to wire this dimmer switch, and even if it is advisable with the way my house is currently wired.
NOTE: the dimmer switch has a knob, and also a push-button.
I have only a vague understanding of all things electrical, so sorry about the vocabulary and long-winded explanation.
Thanks for any help!
Felix
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On 27 Nov 2003, Felix wrote:

Actually, one is the source, the other goes to the fixture, both wires are part of the "hot" side of the circuit, and the switch makes contact between them, as you've noted. More later on why that's important...

Nope. Not even close.
Does your "dimmer" come with a 2nd piece that mounts in the light fixture itself?

Don't take it personally, please, but the fact that you don't know the difference between a traditional "dimmer" and a "remote control device that's hard-wired into a switch box" means that you should leave this work to a professional. There's nothing wrong with trying to learn and grow your skill set, but this job is not the place to start.
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I don't think he was far off, just a bit confusing with terminology.
Many newer dimmers come with three wires. The black and white simply get wired in place of the switch contacts - which is "in" and which is "out" doesn't really matter. The third (green) is riveted to the front plate there to ground the plate (and knob shaft) of the dimmer in case you have a plastic box. Quite important if you have a metallic knob or switch cover plate on a plastic box without integral grounding through the mounting screws.
K&T boxes, of course, aren't grounded, so it's the same thing as a plastic box, only that there's no grounding accessible at all at the box.
I personally would connect the ground to the box (not that it would necessarily do anything), make sure that the knob and plate are plastic, and leave it at that. For insurance, I'd consider wiring in a GFCI outlet "upstream" of the dimmer to protect me from the dimmer (and the fixtures as well). However, in K&T wiring, you never know when a circuit shares neutrals, so the GFCI may not work.
I _suspect_ most inspectors would let him get away with installing the dimmer without the ground wire attached at all.
In any event, he needs to carefully examine the instructions on the dimmer to verify that the green wire is ground. If not, it's a completely different ballgame.
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Chris Lewis, Una confibula non set est
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On 27 Nov 2003, Chris Lewis wrote:



Are you trolling, or are you serious???
There are industry standard color codes for wires. Green is Ground. White is Neutral. Are you seriously suggesting that a manufacturer would code a wire that should be black as a white colored wire?
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Neither wire on a switch is neutral. Virtually every switch ever installed has one white wire on it. You don't even have to mark it in this particular instance, because the NEC assumes everyone _knows_ that any wire connected to a switch is potentially hot.
On a "switch circuit" circuit to a switch, one of the wires pretty well has to be white. But they're both hot. If I remember correctly, the white wire is supposed to be the one "always hot", and the black one is the switched wire one going to the center pin on the lightbulb socket.
Ie: in the fixture, the fact that you have one white wire in a wirenut with one or more other blacks is supposed to be a good hint that that white wire is hot. Secondly, the other wire (which goes to the light bulb) is black to indicate that it's hot.
I have seen simple dimmers with white and black wires. Makes it obvious how to connect switch legs to the switch (even if it doesn't matter), doesn't it?
Most dimmers I've seen are two black wires.
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I agree, Chris. It's a lot more logical to see a white ("known to be hot from here") in a take off for a switch than to see whites on both sides of a load. My code book indicates the conductor must be identified, but nobody does it. For the dimmer, the X10 black/blue system makes a lot of sense.

white
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Thx everyone for all your help.
The green wire on the switch was marked as ground.
I was able to find a ground in a wall socket below so I could test which of the wall wires was hot (or what I call hot).
I connected the white wire on the switch to the "hot" wire. I connected the black wire on the switch to the "other" wire. I connected the green wire to the box, although the box is not grounded.
It all works, and I believe that although it might not be up to 2003 spec, it can't be much worse than the switch that I replaced.
Sincerely, Felix

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