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In DIGI-COMP II an electronic current impulse is simulated by a rolling ball.... In the course of its journey the ball is directed by guides (which simulate the wires which guide electrons in the electronic computer) to the entry points of successive binary digital logic elements. The ball upon entering a logic element "questions" or "interrogates" it for its "state" and possibly changes its "state" in the process of going through it.... With appropriate settings your DIGI-COMP II can be made to:1. Count out a specified number of balls.2. Count in binary arithmetic.3. Add.4. Multiply.5. Multiply and add.6. Obtain either the "1's" or '2's" complement,7. Subtract.8. Divide.9. Zero or clear.10. Overflow.11. Halt.http://www.oldcomputermuseum.com/digicomp_2.html