From Numberphile, science and math writer Alex Bellos introduces the slide rule and then shares “the iPhone of Slide Rules” — the Halden Calculex. On the pocket-sized disk made in Manchester, circa 1906, CollectingMe.com writes:

As described in the instruction booklet, it doesn’t contain any mechanism but only:

“…consists of a disc within a ring which together form a dial with logarithmic scales on both sides, surrounded by a metal ring and protected on each side by a glass disk with a cursor line marked radially thereon, which is capable of being revolved by the two thumbs in order to set the cursor line. The centre of the dial is also turned by holding the nut on each side between the finger and thumb, the ring of dial being fixed to the metal ring.”

Nevertheless, this simple device is not only capable to carry out basic computations, such as multiplication, division, proportion, square roots, cub roots, sines of an angle, but can also be used to perform various complex or combined calculations more easily. For, unlike any other slider rules, most of the basic calculations can be made with only one setting on this one due to the special arrangement of the rounded logarithmic scales.

Other math tools in the archives: the Binary Marble Adding Machine, The Pythagorean theorem water demo, and How Round Is Your Circle?