Important Terms

These are terms that Macdonald came up with to describe his operation of the calculator.

LOCK

As with any 2-cursor circular slide rule, you will need some way to lock the 2 cursors together. Commercial slide rules generally relied on friction between the cursors to hold them in synch.

Within this patent, Macdonald described various mechanical options for the builder which varied based on the size and complexity of the version. His simplest offering was that you place your finger over the vernier AND the index at the same time to hold them together.

For this simple replica, we'll use the last method. You can either press down on top of the vernier (thereby locking the two transparencies together) or resort to my usual go-to: hold them together with masking tape. I have a habit of releasing my finger at the wrong time and mangling the reading. Masking tape is my best friend when I'm testing.

CONNECT

This means hold the index location on the disk (so you can move the vernier). Again, the low-end solution is to hold the Index still with a finger while you operate the vernier, or just tape the index down to the cardboard or foam backing.

ZERO

This means hold the vernier still while you rotate the disk clockwise until the vernier is over the disk origin (i.e., the 1000 mark). Obviously, that's the same as rotating the vernier counterclockwise, but I am following the patent algorithm's method.