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In a rare break with tradition the featured item in this episode of
dustygizmos is totally mechanical in operation, i.e. no batteries are required
(or indeed any sort of electrical supply) to make it work. What makes the Guy’s
Britannic Calculating Machine vaguely relevant on a website largely devoted to
electrical and electronic gadgetry is that devices this were still being made
almost twenty years after the first electronic calculators appeared in 1952 and
ten years after the introduction of transistorised calculators in 1963. By the
early seventies the cost, size and performance of electronic calculators
finally consigned these old hand-cranked marvels to almost overnight
obsolescence.
The Britannic, in common with many other mechanical calculators of
the time is based around an ingenious contrivance called an Odhner Pinwheel,
which in turn was based on mechanisms dating back to the seventeenth century.
More on the exotically named Wilgott Theophil Odhner, the chap responsible for
this revolutionary (pun intended) contraption, in just a moment. 
This Britannic is capable of performing the four most
basic mathematical operations, namely addition, subtraction, multiplication and
division and can handle calculations of up 18 digits in length. Whilst it looks
fiendishly complicated operation is actually quite straightforward. It has
three numeric ‘Registers’ for entering or displaying numbers. The first one –
at the top is the bank of 12 levers. This is the Setting Register, the
equivalent of a keyboard on a modern calculator, where you input numbers, from
0 to 9. Below that, on a sliding carriage are the other two Registers. The one
on the left, comprising a bank of 10 digits is the Revolutions Register which
displays the number of rotations of the large crank handle on the right of the
instrument’s main body. Next to that is the Results Register, a bank of 18
digits, where, as the name implies, the results of calculations are shown. The
silver topped lever is for zeroing the Settings Register and the two small
cranks at either end of the sliding carriage are for zeroing the Rotations and
Results Registers. 
The actual Pinwheel is essentially a barrel, made up of
discs each one having 9 retractable teeth that pop in or out, according to the
number selected by the Setting Register lever. The teeth engage with gears that
drive the numeral wheels on the Results Register each time the handle is
turned.
Anyway, enough of that here’s a simple example of how it works,
and pay attention because you will be tested on it later… Let’s say you want to
add 631 to 294. Step one, move the three rightmost levers on the Setting
Register to 6, 3 and 1. Turn the large crank one rotation clockwise and 631
appears on the Results Register below. Clear the Setting Register by moving the
silver-topped lever upwards and enter the digits 294. Turn the crank one
revolution and the total 925 appears on the Results Register. If you had turned
the crank counter-clockwise the machine would have subtracted one number from
the other.
The Britannic featured here was made by a company called Guys
Calculating Machines of Wood Green in London, possibly in the early 1950s, for
reasons that will be explained later on. It’s hard to say exactly when as the
design changed little over the years but it was popular and apparently produced
in quite large numbers. This appears to be an unsual variant with a 12-digit
Settings Register; most of the others I have seen have 10-digit registers. 