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| + | [* black] Nelson White's U.S. patent "[http://www.google.com/patents?id=lARhAAAAEBAJ|Zero-setting mechanism]", filed in 1920, describes how this amazing thing works. |
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| + | [* black] The clutch rotates one gear forwards when the crank is turned forwards, and it rotates another gear backwards when the crank is turned backwards. |
| + | [* black] One gear turns the upper shaft, the other gear turns the lower shaft. Little arms on the clutch lock the shafts into place when they are not turning. |
| + | [* black] On the left side of the carriage, cams on the upper and lower dials move a lever on the central zeroing shaft. Fingers on the zeroing shaft as a result move into place so that when the upper or lower shaft is rotated, the wheels stop when they display zero. |
| + | [* black] The zeroing mechanism rarely completely fails. It is useful to rotate the crank while observing all the different parts to gain an understanding of each part's function. |
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