The Coin and Letter Scale: A Weight Turned Round Its Pivot

"A is the standard, with the bearings B B, on which the knife-edges x x rest. The knife-edges are firmly secured to the beam C, one end of which has the spring-holder D swinging freely from it on a knife-edge. F is a weight, which is attached to the beam by the pivot G." "Manner of working: The coin to be weighed is placed in the spring-holder, and the weight moved around on the pivot until the mark of the denomination of the coin on the curved plate is directly under the index K on the beam. If the coin is of the proper weight, the beam will be inclined toward the end on which the coin hangs."

**Stand.** Saw an upright 20 cm tall from hardwood board and screw it to a base. File a V-notch on each side of its top with a triangular needle file. **Beam.** Cut 35 cm of aluminium bar. Drill across its middle for a nail; the nail, resting in the notches, is the knife edge. Hang a large binder clip from a wire hook 6 cm from the nail on one side: the holder. **Weight.** On the other side, 4 cm from the nail, bolt a 30 mm strip of the same bar with an M5 bolt, washer and nut, tight enough to stay where it is turned. Bolt flat washers to its far end until the strip weighs 100 g on the kitchen scale. **Mark.** Glue a card semicircle to the beam round the bolt. Turn the weight towards the nail and balance the empty scale level by filing the heavier end of the beam or taping a washer to the lighter end: that is zero. Hang objects weighed on the kitchen scale in the clip, turn the weight until the beam balances, and mark each angle. Compare with the notebook, then weigh five letters and check them on the kitchen scale.

**David Cumming Jr.** of New York received US 39,890, dated 15 September 1863 and antedated 2 February 1863. He wrote that "the machine has been tried and works well, and can be manufactured at a small cost"; his claim is the pivoted weight and the curved plate, operated together. **Honest limits.** A nail in filed notches stands in for his knife edges and bearings, a binder clip for his spring holder, and a tight bolt for his spiral spring. The notebook assumes the empty beam is balanced at zero and ignores the weight's own swing; its sizes are examples. Your marks and checks are the result.