The "PLUS Rapid Adder" is a key-driven adding machine with an
abbreviated or "half" keyboard that was made in England by The Bell
Punch Company from the 1930s to the 1970s. Variations were
manufactured for Sterling and decimal currencies, hours and minutes,
feet and inches, and even tons, hundredweight, and quarters.
The machine is basically a simplified version of the full-keyboard "Comptometer", intended for use in commercial applications requiring only addition and simple multiplication. The "half" keyboard was no disadvantage for these applications, as it was common practice among skilled Comptometer operators to use only the lower half of the keyboard for addition. It was found to be less tiring to enter (eg) 8 by striking 4 twice, than to move one's whole arm to reach the 8 key. The half-keyboard machines were much smaller, lighter, and cheaper than the full-keyboard models, and were entirely satisfactory provided one had no need for subtraction or division.
The machine illustrated is one of several obtained from an accounting firm in Sydney, Australia. The machines were originally purchased in the 1950s for Sterling currency. They were converted to decimal when Australia changed to decimal currency in 1966, and continued in use for another 10 years until replaced by electronic data processing in the late 1970s. The manager of the firm kept his own machine and a number of others in storage for a further 20 years, until disposing of them on his recent retirement.
The notes and illustrations following describe the internal construction and mechanism of a typical PLUS Rapid Adder.