Monroe calculators were first produced in 1912, and, so far as the hand-cranked machines are concerned, didn’t change much in the next 50 years, except that they got smaller.  They work on a different principle to Odhner type machines because they have stepped drums (also known as Leibniz wheels) rather than pinwheels.

Monroe Model G

Serial No. G 34519

Date: 1919 – 1921

Capacity 8x8x16

Price paid: £10.50

This is an early example of a Monroe calculator.  The Model G looks very similar to the later models, but is much bigger (it weighs nearly 15 kilograms).  I have the suspicion that Monroe could quite easily have made their machines smaller in the early days, but decided not to on the grounds that it was easier to sell a more imposing lump of ‘heavy metal’.

In terms of operation, it works like any other calculator, except that the home position of the crank depends on whether addition or subtraction is being performed, and in switching between the two there is a half-turn of dead space in which nothing happens.  This is disconcerting at first, but one soon gets used to it.  One feature of note is the metal button to the left of the keyboard; when turned clockwise it ensures that the adjacent ‘1’ key remains depressed even when the keyboard is cleared.  This means that the left hand part of the result register can be used as a counter with Tens Carry (which the revolutions counter doesn’t have).

The metal knob to the right of the keyboard sets the keyboard so that is doesn’t clear between turns of the crank, as would be required for multiplication and division.  The other key at the front right is the control for manually clearing the keyboard.

The advantage of these machines over the pinwheel types is that numbers can be entered more than one at a time, so leading to greater efficiency (or so the Monroe salesman would claim).  In practice, the tendency is to enter the numbers one at a time.

A quirk of this machine, when compared with later models, is the carriage doesn’t automatically lift when the registers are cleared, so in order to clear the registers the carriage must be lifted manually by turning the crank on the front of the machine a quarter turn.

I think I got lucky with this machine because it is quite rare and was inexpensive. It is complete apart from the fact that the decimal point markers on the carriage have been lost, and that the keyboard reset button was missing.  I fortunately managed to have a replacement made – thanks, Cris!

The machine works well, but is most reliable when the turns of the crank are smartly executed.  I have no idea how much internal friction is normal on these machines, and while I wouldn’t exactly say that the machine is stiff to operate, it requires quite a bit more effort than the L Series machines described next.

Monroe Model L-160X

Serial No. 520372

Date: 1940s

Capacity: 8x8x16

Price paid (including postage): £50.74

Essentially a smaller version of a Monroe calculator – it weighs a mere 4 kilograms.  The ‘X’ in the model designation stands for ‘executive’.  This type of machine would have been equally suitable for use in the field as in the office.


Monroe Model L160-X

Serial No. 577279

Date: 1950s

Capacity: 8x8x16

Price paid (including postage): £25

A later model in grey.  This machine came in a dedicated case which contains a metal bracket to which the machine is clamped during transport.

Monroe Model LA5-160

Serial No. 211181R

Date: 1930s (?)

Capacity 8x8x16

Price paid (including postage): £41

This machine is electric, and performs automatic division by means of repeated subtraction until an underflow is detected, then correcting the underflow with a single addition, then moving the carriage one place to the left and repeating the process in the next decade, and so on.

I run this machine with a 110 volt supply while wearing rubber shoes.  Just kidding about the shoes, but I’ll bet it wouldn’t pass a PAT test.

The holes in the front plate adjacent to the leftmost ‘1’ key is where a clip would have been attached to hold the ‘1’ key down during multiplications in a similar way to the knob on the Model G.  This clip is invariably missing, and this machine is no exception.

A video of this machine performing the division 355/113 (=3.1415929) can be seen by clicking the link below.

Video of the LA5-160

One thing I’ve noticed about my Monroe collection is that on all of my machines, including the Model G, all of the keys are the same colour, whereas most of the pictures on the internet show the keys in two different colours.  I’m wondering whether this is a peculiarity of machines sold to the British market.

Monroe Model SS-160 (Slide Set) 

Serial No. 215886

Date: 1930s

Capacity: 8x8x16

Price paid: £26

This is an interesting machine, based on the Model L, and is believed to be somewhat rare.  It is fairly close in serial number to my LA5-160, which seems to place it in the 1930s.  The slide input mechanism was patented in 1934, but seems not to have been a success.

Advantages of the slide input are that it is easier to see the number about to be input.  The calculator is also more suitable for pinwheel calculator type algorithms for calculating square roots etc.

Disadvantages are that number entry is slow (thus negating the chief advantage Monroe had over pinwheel machines), and that compared with pinwheel machines the numbers are entered in the opposite sense i.e. the slides move up rather than down.


Monroe “Monroematic” Model 88N-213


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