Arithmometer

The Odhner Arithmometer

The Odhner arithmometer (patented 1874 by Willgodt Odhner) is a pinwheel mechanical calculator. The pinwheel mechanism uses a rotating drum with a variable number of teeth to perform arithmetic.

How It Works

The Odhner uses pinwheels — each wheel has 9 retractable pins on a disc. Setting a digit extends the corresponding number of pins. When the drum rotates, the extended pins engage the counting mechanism.

Components

• Input register — where you set the multiplicand/dividend
• Result register — accumulates the result
• Revolution counter — counts rotations (tracks multiplier/quotient)
• Handle — turn clockwise to add, counter-clockwise to subtract

Basic Operations

Addition: A + B

1. Clear all registers
2. Set A in the input register
3. Turn handle clockwise once
4. Set B in the input register
5. Turn handle clockwise once
6. Read result register

Subtraction: A - B

1. Clear all registers
2. Set A in the input register
3. Turn handle clockwise once
4. Set B in the input register
5. Turn handle counter-clockwise once
6. Read result register

Multiplication: A × B

1. Clear all registers
2. Set A in the input register
3. Set carriage to units position
4. Turn handle clockwise B times (for single-digit B)
5. For multi-digit B: handle × units digit, shift carriage, handle × tens digit, etc.
6. Read result register

Division: A ÷ B

1. Clear all registers
2. Set A in the result register directly
3. Set B in the input register
4. Turn handle counter-clockwise until bell rings (indicates overflow)
5. Turn handle clockwise once to correct
6. Shift carriage, repeat
7. Read quotient in revolution counter, remainder in result register

The rhythmic clatter of the mechanism and the elegance of purely mechanical computation make the arithmometer a pleasure to use.