As described by Reuters, you must bring the string to the appropriate temperature to produce the "right" pitch. This means that you must keep it at the "right" temperature in order to keep it tuned, so the power would have to stay on until you've finished your performance. Of course, since the heat will leak from one string to the next, and to the soundboard, the "right" temperature will change continuously; so we'd have to assume a "closed loop servo" configuration that constantly "tracks" the pitch of each string and adjusts the heating current as required.

There's no way that any reasonable rate of heating could bring a 300 pound soundboard to "stable/constant" temperature in 40 seconds. As the soundboard warms and expands, it will increase the string tension, leading to an additional requirement for heat to the string - - at least until "stable" temperature gradients are achieved, "active" tuning would need to continue.

While it's not difficult to generate any particular frequency, or to compare it to another frequency, the existence of the "other frequency" is a bit problematic. You can't tell at what frequency the string will vibrate - unless it vibrates. So does the piano "hum" for you the whole time the tuner is adjusting itself???? And do you have an 88 note drone accompaniment until the system is stable???

There is a "tuning problem" to which this system might have some application. Large concert halls are frequently not kept "at temperature" except for performances, and the heat load from a large audience can change the room temperature significantly; so the piano tuner may have to work at an ambient temperature significantly different than the temperature that will prevail for the performance. In the rare instance where this is a known problem the proposed system might have some merit - if it actually works.

For the banjo player problem, the solution is simple. If the string is out of tune, heat it 'till it burns their fingers. Or just accept and enjoy the characteristic "unpitched twank" they make. It's really part of their charm.

John