| 摘要 |
A balance for teaching arithmetic principles has a balance arm pivotal about a central horizontal pivot axis coincident with the axis of a support rod supported in openings at the top of opposed vertical upstanding members of a support. The balance arm has a central rectangular housing and end rectangular housings at each end extending upward carrying vertical calibrations at intervals corresponding to the height of annular washers that balance the arm. A number of upstanding vertical pins similarly vertically calibrated are spaced at one-inch intervals on either side of the pivot axis at horizontal locations on the arm along respective horizontal calibration indicia of diameter corresponding substantially to the diameter of the central openings in the washers. The front face of the balance arm is transparent along its length for exposing scales on a central mandrel carrying a number of scales that may be rotated to expose a selected scale. The pivot axis is at a height corresponding to the average center of gravity of a typical vertical pile of washers, typically at about the height of 5.5 washers above the base of the vertical pins. The balance arm may have a back plate calibrated both vertically and horizontally with the central rectangular housing formed with a tapered opening for accommodating an upstanding tapered member on which the balance arm pivots.
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