The Guided-Dish Letter Balance: The Same Reading Anywhere on the Dish

"As usually constructed, the dish a of the letter-balance is affixed to the beam b at or near its centre. When so affixed or attached, the dish is apt to tilt when any weight or letter or package to be weighed is placed near its edge or corners, so that in the hurry and dispatch of business the weighing of the letters and packages will be imperfect and incorrect." "From the middle of the fork a rod, +, passes out towards a vertical rod, f, attached at its upper end to the centre of the dish, and its lower end jointed to a horizontal vibrating-rod, which is also jointed to a rod placed vertically, and permanently fixed inside of the base, g."

**Post.** Saw an upright 20 cm tall from hardwood board and screw it to a base. Drill two nail holes through it, one 3 cm below the top and one 9 cm below it. **Beam and link.** Cut two strips of aluminium bar: the beam 30 cm long and the link 8 cm long. Drill both for nails 8 cm apart at one end, so beam and link are the same length between holes. Pivot the beam on the top nail and the link on the lower one. **Hanger and dish.** Cut a hanger strip 7 cm long and nail it to the ends of beam and link so the four pivots make a parallelogram. Screw a plywood disc cut with the hole saw on top of the hanger: the dish. **Scale.** Clip a large binder clip loaded with washers on the long end of the beam as the sliding weight. Mark the beam by hanging objects weighed on the kitchen scale on the dish's centre and sliding the clip until level. **Test.** Weigh one letter at the dish's centre, at its front edge and at its back edge. Then tape the hanger rigidly to the beam, take the link off, and weigh it at the same three places. Compare with the notebook.

**Benjamin Chambers Jr.** of Washington, D.C., signed US 75,244 on 30 January 1868; it is dated 10 March 1868. His beam was cast in one piece with its fork, its knife edges sat in holes in lugs and uprights with small covering plates, one pivoted, so the beam could be lifted out, and a screw nut trimmed the balance to zero. His claim is those covered knife-edge holes. **Honest limits.** Nails in drilled holes stand in for his knife edges, and a nailed parallelogram for his rod and link. The notebook treats the fixed dish as a rigid extension of the beam; its sizes are examples. Your six weighings are the result.