Comptometers are key-driven machines – that is, the act of depressing a key adds the appropriate number to the running total (although in fact the number is actually entered on the return stroke).

Strictly speaking, the term comptometer should only be used for machines made by Felt and Tarrant because it was registered as a trademark by them.  However, much to Felt and Tarrant’s presumed annoyance, the term came to be generally applied to any machine of this type.  In a twist of fate, however, after Felt and Tarrant ceased production in the late 1950s, the Bell Punch Company, a long time competitor, bought the rights to the comptometer name.

Numbers can be input very quickly because a parallel data entry system is employed.  In other words, multiple digits can be entered in a single action by depressing the keys of several columns simultaneously.  This is in contrast to modern calculators which employ sequential number entry.  Because of this speed advantage, comptometers continued to be used into the 1970s until computerised systems rendered them obsolete.

It is noted that the first ever electronic calculator was built to mimic the operation of a comptometer, but was not a success because it had no real advantage when compared with the mechanical equivalent, and cost much more.

That said, mechanical comptometers were expensive items in their day, and were commonly leased to companies rather than sold outright.  The terms of the lease would usually cover servicing and maintenance.

A comptometer can only add numbers.  However, a number can be subtracted by adding the complement of the number.

Multiplication can be undertaken by repeated addition, so 123 multiplied by 9 would be calculated by simply depressing the keys 123 nine times.  More complex multiplications can be performed by adding in different decades.  For example, 144 multiplied by 12 would be calculated by adding 144 twice, and 1440 once (by simply moving all of your fingers one position to the left.

An expert operator would be able to perform division and extract square roots, but these tasks would usually be better undertaken by other means.


Felt and Tarrant Model J

Serial No. 261557

Date: 1926

Price paid (including postage): £33.51

Felt and Tarrant comptometers are easy to date because the first two letters of the serial number indicate the year of manufacture.

This particular example is a U.S. made Sterling currency machine built for the British market.  For those unfamiliar with the Sterling currency system, it’s probably better to stay that way; suffice to say that the pound Sterling (GBP) was divided into 20 shillings, each of 12 pennies.   Pennies were further subdivided into halfpennies (half of one penny) and farthings (quarter of a penny).  There were further archaic subdivisions of the pound into crowns (5 shillings), half-crowns (2 shillings and 6 pence – yes, there was a coin of this denomination!) and florins (2 shillings).  On top of this, there were informal designations for various coins.  And let’s not even think about imperial (pre-metric) weights and measures.

On this machine, whole pennies (this machine doesn’t handle fractions of a penny), are entered via the rightmost column of keys, numbered 1 to 11. Shillings are entered via the next column to the left for values from 1 to 9, and the single key in the third column for a value of 10.  The remainder of the keys are used for entering pounds and tens of pounds etc.

The machine locks up if a key is not fully depressed during number entry.  The red button at the right rear has to be depressed to release the keys before data entry can continue.  Another interlock prevents two keys in the same column from being depressed simultaneously.  The lever on the right side of the machine is used to reset the machine to zero.

A subtlety of Felt and Tarrant machines in particular is that following a reset, the numerals displayed in the number windows are displaced slightly upwards.  This is a visual indication that the machine is ready to start a new calculation.  Also, upon the first entry following a reset, extra resistance is felt and a bell rings.

This particular example is in working order, but numbers in the second column from the left need to be entered with gusto.


London Computator Corporation (1941)

Serial No. LC/912/S/2640

Date: 1941

Price paid: £25

This is a very early example of a British machine made by The Bell Punch Company Limited (who also manufactured ticketing machines which gave the company its name).  The machine is similar in operation to the Felt and Tarrant machine described above (although the internal mechanisms operate on different principles).  However, the numerals are not displaced upwards following a reset.  Another difference is that the pence (pennies) column only runs from 1 to 9, meaning that entries of 10 and 11 pence require two consecutive key presses in that column.  The slight inconvenience to the operator is therefore over-ridden by economies of manufacture.

With regards to the serial number, the 9 refers to the number of keys in each column, and the 12 refers to the number of columns. The S refers to Sterling currency.

The machine is painted in a crinkle finish green.  The machine was filthy when bought at auction. There used to be very faint evidence of the Sumlock name in white on the front panel, but this unfortunately disappeared during the removal of several decades worth of tobacco residues.

The machine is in good working order, and was very easy to date because of the presence of the following tag on the underside:

Calculators 008

It can be stated with certainty that the machine was manufactured before 11 July 1941.

Sumlock (1940s)

Serial No. 912/S/21.797Q

Date: 1940s

Price paid (including postage): £32.99

The machine shown is functionally identical to the previous machine.  Clearly, at some time following 1941, Bell Punch started branding their machines as Sumlock.  I don’t know whether the internal mechanism is the same as on the early machine, because I haven’t looked.

The machine is painted in a dull green gloss.  It was probably shiny in its heyday.

The machine is in good working order.


Sumlock (1950s)

Serial No. 912/S/113.092

Date: 1950s

Price paid (including postage): £20.50

Again, functionally identical (and mechanically, too, for all I know) to the earlier machines by this company.  The previous keys, comprising plastic key caps atop flat metal key stems have been replaced with cylindrical plastic keys projecting through circular holes in the cover plate.

The outer case is painted in that green hammered finish which is so redolent of the late 1950s.

The machine is in good working order.


Sumlock Comptometer (1960s)

Serial No. 912/IVB/S/146.975

Date: 1960s

Price paid (including postage): £15.49

Following Bell Punch Limited’s acquisition of the comptometer name from Felt and Tarrant, Bell Punch could now market their machines as comptometers (and even export them to the United States, where they were marketed as ‘new’ machines).

The machine is finished in a rather fetching pale pink.

It is in good working order.

Firm evidence for the date of the machine comes in the form of a servicing chit:

Calculators 011

It required a lot of training to become a competent comptometer operator.  After training, an operator’s badge would be awarded:

Comptometer Badge


Electric Comptometer


Sumlock Duoelectric 

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