Otis King

This is a great design for higher precision multiplication and division. By wrapping the logarithmic scale around a cylinder it can be a lot longer than when laid out along the edge of a ruler. Longer means you can pack more tick marks in. Ten times the length means ten times the tick marks means an extra digit of precision.

Good information about Otis Kings can be found at Otis King's Patent Calculator.

Model K, probably dating to the 1960s.

Model L, probably dating to the 1950s

The prior owner added terse instructions for multiplying and dividing (B=bottom indicator, T=top indicator, unity=1 on the scale):

MULTIPLY:

1. B to multiplicand

2. Unity to T

3. T to multiplier

DIVIDE

1. B to dividend

2. Divisor to T

3. T to unity

ANSWER at B

How to Disassemble and Clean an Otis King

[NOTE: These instructions apply to "late model" OKs. I don't know when OKs with this construction started being made, but there is at least one earlier form construction. Look for an article in the Spring 2012 edition of the Oughtred Society Journal where I document how to disassemble that earlier kind.]

Easy use depends on everything being clean. But how do you open an OK to clean it? Or what if you need to replace the scales? (You can get pdfs for them at The International Slide Rule Museum.)

1) Remove the knurled top, which unscrews counter clockwise. (Note that it may be very tight and/or seized.)

2) Separate the cursor body from its lower ring. The white dot and the unmarked dot on the opposite side are just retaining dimples. I separated the body from the ring by simply extending the upper cylinder, sliding the cursor fully upward, and then really pulling it hard further up. With enough yank, the ring separates.

3) Pull the cursor body up and then see that there is a felt-ringed, threaded brass cap screwed to the top rim of the lower cylinder. Remove the cursor body over the top end and then unscrew that cap (counter clockwise) and remove it too.

4) Now you can pull the upper cylinder out of the lower one. The bottom of it is felt-wrapped. You will also find a slot through the bottom of the top cylinder, through which slides a steel "key" to prevent the cylinder from being pulled past the threaded cap when all is assembled.

To reassemble, put the cursor ring on the bottom cylinder, stick the key in the top cylinder slot, and slide the top into the bottom. Then slide the threaded brass cap down over the top and screw it to the upper rim of the bottom. Slide the upper part of the cursor over the top, line up the dimples with the receiving dimples in the bottom ring and jam them together. Screw the knurled top back on and you're done.