Pinwheel Calculator

Before the introduction of the electronic calculator in the late 1960s, the equipment of choice for dealing rooms was the Pinwheel Calculator, which was also used in the back office to make sure the dealers got their calculations right. It was a mechanical calculator that could handle multiplication and division as well as addition and subtraction. This ingenious piece of equipment was based on the Arithmometer, a machine patented in France in 1820 by Thomas de Colmar and developed into a marketable product 70 years later by Willgodt Odhner, a Swede working for Nobel. From then until the late 1960s, that machine and its clones became the calculator of choice in business and science.

When joining a dealing room at that time, one not only had to learn the intricacies of the market, but also to master this intriguing gadget. The main parts of the machine were the pinwheels, a series of geared wheels, each with a small lever to slide down to set the numbers to work with, the large crank handle to rotate that number onto the upper of the three numerical registers that displayed the results and the carriage at the bottom, which could be moved left and right to input the numerals in large numbers in the right-hand register while the rotations were counted in the left.

Addition: Enter the first number to be added by sliding the levers down to the value of that number, then rotating the large crank clockwise once so that the number winds into the top register. Set the second number on the levers, rotate the crank clockwise again and that number is added to the number in the top register. Although simple, this was fiddly and, in fact, it could be quicker and easier to do this with pencil and paper.

Subtraction: Similar to addition except, having input the first number in the top register, the number to be subtracted is set on the levers and removed from the number in the top register by rotating the crank anti-clockwise. Again, it was just as easy to do this on paper.

Where the machine came into its own was in Multiplication and Division, particularly of long numbers. However, this was not complicated, it was merely a matter of repeating the action of addition and subtraction. To multiply, keep adding the number to itself by cranking the handle clockwise the number of times you want to multiply by and, to divide, by subtracting the divisor from the dividend by cranking anti-clockwise until you reduce the dividend in the top register to zero. The bottom left numerical register, in counting the rotations, displays the quotient. A refinement that obviated the need to crank the handle a million times, if multiplying, say by a million, was provided by the carriage mechanism, which could be shifted left or right to add or subtract the tens, hundred, thousands, etc.

The average time for conducting long multiplication and division calculations was 30 and 45 seconds respectively and operation of the machine was noisy. So the electronic calculator that succeeded the pinwheel was very welcome.

With thanks to Alan Baker, ex Rothschilds, Sharps Pixley, Deutsche Bank and former Chairman of the LBMA