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COMPTOMETERS

These could be faster than an electronic calculator because all the digits of a number could be entered at the same time.

This machine, invented by Dorr Eugene Felt in America in 1887, was designed mostly to add up and subtract, but it was capable of multiplication, division and more complex calculations, and some were designed to deal with old British currency (pounds, shillings and pence). They were in regular use until the 1970s and were often so well built that the manufacturers went out of business because spare parts were available and they didn't need to sell replacements.

The way they were used was similar to the calculating machine, with the advantage that all keys for one number could be pressed at the same time, and so was much faster.

The key board

The first row of keys on the left represented units 1, 2, 3, etc

The second represents 10s: 10, 20, 30 etc

The third represents 100s and so on.

This meant that it wasn't necessary to press 1 and 0 for 10 as we do today: there are no buttons for 0.

For example, to enter the number 405, press 5 in the first row, and 4 in the third row

Adding

Just pressing any key would add the value of that key to the total at the top of the machine. It wasn't actually necessary to press the keys for a long number in the correct order.

Multiplication

To multiply 8 x 4 press the number 8 key 4 times

To multiply 23 x 45 press 23 five times then move the fingers along one set of keys (to the tens) and press them four times.

Other calculations were more complex and the operators had to be well trained to do these.

This electrically powered example, made by Friden of America, is a sophisticated comptometer. It has + and – buttons which were pushed after each number was entered and a multiplication pad on the bottom left-hand side.

There are detailed descriptions of this machine on the internet.