If memory serves, the Baldwin bubble gauge vial is calibrated in increments that indicate 17 minutes of arc. I think this gauge was designed by Harold Conklin and he set up the calibration this way to make it easier to translate string deflection into string downforce against the bridge. Using this gauge, if the bubble is zeroed on the speaking portion of the string and it indicates a positive deflection of one gradation line (or 17 minutes of arc) on the back scale portion of the string, the downforce of that string against the bridge will be 0.5% of the scale tension of that string. (The length of the backscale does not matter, only the angle of deflection.) Two gradation lines indicates 1.0% downforce, etc. I.e., if a single string has a scale tension of 170 pounds and the gauge indicates two gradation lines of positive deflection, the downforce of that string against the bridge will be 1.70 pounds. If the total scale tension of the piano were 40,000 pounds and the bubble gauge indicated two gradation lines of deflection on each string the total downforce would be 400 pounds. I’ll leave it to the mathematicians among us to figure out what the gradation lines of the Lowell bubble vial indicate. ddf
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