Hi I have been noticing a burning smell from one of the walls in one room of the house.I called the electrician to take a look at it but he could to figure it out as I could not reproduce it during day.Apparently it happens only at night when I switch on the lights of that room for a prolonged period 10 -20 minutes.Any ideas about what could this possibly mean and how to get it diagnosed as it seems to happen at night

Go to Harbor Freight and get a laser thermometer ($30 or so)
Don't worry, you will find other uses for it (cooking, working on your car, checking the fridge or A/C etc)

When you think you are smelling something, point that thermometer at every outlet and switch. It might help to take the cover off but don't go in any farther. If you find a hot spot, call the electrician back and tell him what you found.

I would call an electrician, explain that it happens with the lights on and ask if the electrician has an IR scanner/camera.

Another call could go to the local Fire Dept, they probably have the IR equipment, and should be happy to help you before you have to call them 911.