This green plastic ruler was an unexpected flea market find, and I haven’t seen anything like it before or since.
The ruler is 27.5 cm long, made of green plastic that looks positively cheap, suggesting 1950s or 1960s manufacture. The two sides bear different scales; one is marked + and –, the other X. The sliding runner has חשב (Cheshev) molded on both sides; the Hebrew word derives from the root for “Calculate” or “Think”, an apt brand for a calculation aid.
This is clearly a simple calculation aid, the kind you might give to school children. It embodies a 9×9 multiplication table on one side and a 9×9 addition table on the other. You operate it by sliding the runner until the two operands are in the outer holes, and then the result appears in the middle hole. The photos below show the setting for calculating 9 x 3 =27 and 6 + 4 = 10.
Note how the 9×9 span was covered in 36 positions along the ruler. Obviously, this is achieved by avoiding redundant calculations that transpose the same operands, e.g. 2×6 and 6×2, and avoiding multiplication by 1.
That’s all for this item… unless someone comes up with more information about who made it and exactly when. If you know, do comment below!
Exhibit provenance: the Jaffa flea market.
More info: Here is another calculation aid serving the multiplication table.