Even though now we are satisfied that these two coils are each one foot long, they probably won't stay that way. After they're adjusted, the coils still may change slightly. If we return in a couple of hours, the coils won't be exactly one foot long, because they haven't completely adjusted to their new position. It may take a day or a week for that to happen. If the piano hasn't been tuned frequently enough, a technician could spend all day tuning and the piano will still go out of tune, because the wires have not had time to settle into their new positions.
Humidity refers to the amount of moisture in the air.
Humidity plays a major role in why pianos go out of tune. This is because humidity affects wood, and pianos are made of wood. Seasonal and even daily changes in humidity cause wood parts to swell and shrink, affecting tuning stability and touch. It has a slightly crowned shape. You can picture the soundboard as a thin membrane that vibrates up and down when it receives sound waves.