Monroe

Monroe LA6-200

This model runs on AC, but it's just motorized. Calculation is strictly mechanical via a pair of stepped gears in each column.

One wider gear has 4 teeth in a stepped arrangement. Pressing the 1 or 6 key shifts it laterally to bring only 1 tooth under a transfer gear. Pressing the 2 or 7 key shifts it a little more so 2 teeth are under the transfer gear. Similarly for keys 3, 4, 8 and 9. Pressing the keys for 5 and above moves a separate gear with 5 teeth under the transfer gear. The motor turns all the gears at the same time and the transfer gears turn the accumulator dials.

If an accumulator dial rolls over 9 during addition a finger between the columns is raised. After the gears all the dials have been turned by the transfer gear, set of stepped detectors on a drum rotate past the fingers. Because the detectors are stepped, the first finger to be felt for is the one between the 1s and 10s column. If the detector feels that finger raised, it advances the 10s dial one digit. Next, the detector for the finger between the 10s and 100s column rolls by, and if that finger has been raised either by the initial transfer of values from the keyboard or by the immediately prior detection of the finger between the 1s and 10s, the 100s dial is turned one digit. In this way carries ripple leftward, but only for 11 places. Carries for the leftmost 9 places are via "odometer" style rollover.

The gears/drum rotate one way for addition, and the other for subtraction (although the motor only turns one way.) It sports a counter register whose digit in whatever place corresponds to the accumulator's laterally shifted position is incremented for every addition and decremented for every subtraction.

The big red button at lower right clears the keyboard (raises all the keys). The button above it (missing an "R" keytop) when pressed engages a "reset mode" that automatically clears the keyboard at the end of an addition or subtraction. (The orange key above it disengages reset mode and raises the R key.) This is great if you're adding or subtracting one number after another but not if you want to do multiplication (division) by repeated addition (subtraction). There's also a little silver pin on right side which, when pressed in, engages reset mode if the big clear button is pressed. I guess the idea is that after hitting clear you're most likely to want to be in reset mode thereafter. Pulling the silver pin outward allows the keyboard to be cleared without exiting reset mode.

The device also supports multiplication and division. For multiplication, accumulator is zeroed and the shorter of the two numbers (the multiplier) is added to the rightmost digits of the accumulator. Then the accumulator is shifted rightward until the most significant digit of the multiplier (now in the accumulator) is sitting just to the right of the 1s column. The other number (the multiplicand) is then entered on the keyboard, and the red multiplication lever (at top right) is engaged. The calculator then does an addition, and at the end it a little finger at the extreme right edge of the device decrements the most significant digit (MSD) of the multiplier.

If the MSD doesn't decrement to zero, the machine does another addition. But if the machine detects that the MSD is zero, it triggers the machine to automatically shift the accumulator leftward one place! The multiplicand is then added into the shifted accumulator as many times as the 2nd digit of the multiplier, and when it goes to zero the accumulator is again shifted left. This proceeds until the multiplier's digits have been exhausted and the accumulator has been shifted left as many digits as were in the multiplier. Thus, the product 1234x567 is really accomplished by 5 additions of 123400, 6 additions of 12340 and 7 additions of 1234!

Division is done by adding the dividend into the cleared accumulator, clearing the keyboard, entering the divisor and shifting the accumulator rightward so that the divisor is aligned under whatever leading part of the dividend it is less than. (E.g. if you were dividing 1234 by 9 you would add 1234 to the cleared accumulator, enter 9 on the keyboard, and then shift the accumulator so that the 9 on the keyboard was below the 2.) The red division lever (top left) is then engaged and the machine does a subtraction. When the leading digit of the accumulator goes to zero, the machine shifts it left and continues. Eventually the counter shows the quotient and the accumulator only contains the remainder.

I restored this machine from partially working condition. The mechanism for automatically shifting the accumulator didn't work at all, and neither did the accumulator's clearing feature (the crank at the right end of the accumulator, which is turned one way to clear it and the reverse way to clear the counter.) After cleaning and a little work I got the shifter working, but a part of the accumulator clearing mechanism has broken off and I could only get partial functionality out of it. The calculator is also missing a few keytops.

Monroe Educator

This one isn't motorized and if you want to multiply or divide you have to repeat the additions/subtractions and move the carriage manually. Turn the crank on the right one way for addition, the other for subtraction.

This unit is missing a tooth on the transfer gear of the 1s column. Consequently, the accumulator dial above the 1st column can't be advanced past that tooth. Other than that, it's operational and in good condition.