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Operating a "Comptometer"

Instructions for basic arithmetic operations with a "Comptometer" type calculator.

This is a new article and was not in "The International Calculator Collector".

These instructions for performing the basic 4-functions on a "Comptometer" type calculator are taken from the operating instructions of the Sumlock machine manufactured by the Bell Punch Company, though they can be applied to Comptometer-type machines in general.


A typical "Comptometer" calculator with a "full-keyboard" is shown below:

Note that each key of this "full-keyboard" machine has two numbers. The larger-sized number is used for addition and multiplication, whereas the smaller-sized number (which is the "nines complement") is used in subtraction and division.

The little buttons at the bottom of each column of keys are the "subtraction buttons".

The Bell Punch Company specialised in simpler machines with "abbreviated keyboards", as illustrated below.  These only have keys up to 5 in any column, do not have the small numbers on the keys or the subtraction buttons, and were designed mainly for addition, as described below.  

Comptometer with abbreviated keyboard

Comptometer operators were taught to use their machine in the same way that typewriters and computer keyboards can be operated using the "touch typing" technique.  Here you do not look at the keys but move the fingers to the correct keys using touch.

An important feature of Comptometers, which is widely used in these instructions, is that keys in different columns can be pressed simultaneously.  So to enter 123 the fingers of one hand are placed on the 1, 2, and 3 keys of adjacent columns and then all fingers are pressed at the same time.



"It is important that the operator does not look at the keyboard when adding.  Keys Nos. 1 to 5 only are used in addition.  This enables greater speed as it is quicker to strike two keys with the hand practically stationary than to move the hand to a distant key.

The odd numbered kays 1, 3 and 5 are hollowed while those of the even numbers 2 and 4 are flat.  This enables the operator to obtain a greater speed by the "touch" method.

The machine is operated by pressing one key [in any column] at a time.  Should two keys in the one column be accidentally pressed the lower key only will register.

As mentioned previously keys numbered 1 to 5 only keys are used.

The answer is shown in the glass panel at the foot of the machine.  To clear the figures pull the Clearance Lever forward and release."




"Large figures on the keys are used for multiplication and the full complement of keys 1 to 9 in each column are used.

The operator must remember to press the keys simultaneously with a firm stroke and even action.

a) Multiplication of whole numbers.

When multiplying whole numbers move from right to left of machine in the same manner as is done on paper.

b) Multiplication of decimals.

When multiplying decimals move from left to right of machine, also working from left to right of the figures being multiplied.




"RULE.    USE SMALL NUMBERS ON THE KEYBOARD LESS 1, i.e. when subtracting 73, depress small 7 (in the tens column) and the small 2 (in the units column).

Always depress one key at a time, starting with the left hand figure of the amount to be subtracted.  When "0" occurs between other figures as in 2023 always depress the small "0" in the column in which the 0 appears.

When "9"s occur with other figures as in 993, do not depress any keys in those particular columns as there are no small "9"s on the machine.  They are of no value.  When subtracting say 4200 in making this one less, 4199 the only keys to depress are the 4 (in the thousands column) and the 1 (in the hundreds).


SUBTRACTION BUTTONS.   The row of buttons ranged below the keys and above the answer dials are the Subtraction Buttons.




"The small figures on the keys are used for division and like subtraction one is deducted from the last figure of value.

There are three rules to be carried out in division:

  1. Catch up with "Index" Figure,
  2. Reduce remainder,
  3. Move over.

The Index figure is the figure in the dial immediately to the left of the figure or figures the operator is holding.

The remainder is always the amount in the dials immediately underneath the columns where the divisor is being held.

The operator must always mark off first. Place the decimal pointer in its correct place, moving one place to the left for each whole number

Always check division by multiplication i.e. Divisor X quotient = Dividend.

When the divisor does not go into the dividend the exact number of times, it will be necessary to divide right across the keyboard."



After reading these instruction I feel very glad that electronic pocket calculators were developed.  Subtraction and division appear to be very prone to errors if the person lost concentration.  You can appreciate why there were may "colleges" teaching Comptometer techniques run by the manufacturers.


For comprehensive instructions on using decimal Comptometers see the book "How to use the Calculator and the Comptometer" by James R. Meehan, 1952.  This is available to download at

Comptometer Educator

The photograph above shows a "Comptometer Educator", which is a dummy abbreviated Comptometer used for training purposes.  It is used to practice entering complete numbers in one go using all the fingers of the hand.

Comptometer school

This photograph is from the journal "Office Magazine" for January 1955 and has the caption "Over 180 girls a year pass through the Liverpool school for Comptometer operators run by Felt & Tarrant Ltd.  Course normally lasts three months, but there is a shorter course, provided free, which covers three weeks and deals with one specific application of the Comptometer".

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© Text & photographs copyright Nigel Tout   2000-2017  except where noted otherwise.