Description and Notes:
This circular slide rule comes with its original leather wallet and the 95 page soft leather back instruction manual, which also contains numerous rules, tables, and formulae to facilitate the use of this particular slide rule. The wallet is well worn as shown in the picture, but it has served its purpose very well, since the slide rule itself is in a pristine, almost like new condition. This rule is dated to the first quarter of the twentieth century because its appearance is exactly the same as the one shown on pp. 88 and 89 of Halden's 1914 catalogue (more pages of the catalogue can be found in the FTL Design website).
The construction of the Halden Calculex is rather simple comparing to the other three circular slide rules in this collection, namely the Calculigraphe, the Fowler's "Magnum" Calculator, and the Russian KL-1. 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.