Spiral mechanical calculator, The Adal Calculator, metal, The Addall Company, Temple Courts, Temple Row, Birmingham, 1910.
This object consists in a pair of metal disks and a cursor. The base is a flat aluminium disk with the numbers 00 to 99 around an outer ring which forms a lip. Concentric with this disk and laid upon it, is a thin brass disk which has one hundred small semi-circular indents and the numbers 00 to 99 in a ring around it. The main part of the upper disk is formed into a spiral of 11 turns. There is a tongue of brass attached to the central bolt and having a slot in which a steel ball slides as it accumulates turns of the spiral disk. The slot of the tongue has the numbers 1 to 11 marked on it at intervals equivalent to the step between each turn of the spiral. At its further end is a small clamp which holds it at the zero point of the outer ring so that it acts as a stop for the rotation of the accumulating spiral disk.
The whole calculator is held in the flat of the hand. It is a simple adder to 1199 with addends 1-99. The spiral disk is turned by a stylus set into the indent of the number to be added until it reaches the stop. As each number is added the spiral disk rotates and the small steel ball slides in the slot in the tongue indicating the hundreds count of the accumulated result. The total result is thus the number indicated on the tongue (being the hundreds digit) plus the number indicated in the end gap of the stop.