[torbjorn]

That also depends on how the multimeter is designed. Unlinearities in the input circuitry can cause a measurable DC offset voltage when the multimeter is switched to DC and subjected to a high AC voltage. Try changing the polarity of the multimeter and see if the indicated DC voltage's polarity does change or not, if it doesn't change, then the DC reading is only due to a badly designed multimeter.

In some cases, small DC voltages may be superimposed on the AC mains if there is equipment connected to the mains that do not draw equal amounts of current during the positive and negativ halves of the AC waveform. For instance, there are some heating appliances that can be switched to half power by a diode in series with the heater, furthermore we have badly designed dimmers and similar devices. In the EU, there is now legislation and ordinances that require that devices to be supplied from the AC mains have to draw a symmetrical current. But there are still many older devices in operation. I'm not sure wheter there is such legislation in the USA or other parts of the world.

[Guest]

It's a residual voltage. More so along the lines of being 'noise.' That's what rl and rc filters are for. They suppress/filter and stray ripple.

[daddeeee]

why does an ac voltage source showing dc voltages also when measured with a multimeter?