Make Fearns, UK
Model Circular Slide Rule

1. This rule was made by a British company who, from information on the instruction leaflet, also produced a range of specialised circular rules.
2. One interesting feature of this rule is the spiral log log scale. The starting and end points are highlighted below. The range went from 1.01 to 1 million. This is a wider range than many straight slide rules.
3. This rule shows at the same time one of the advantages and disadvantages of circular rules. The outer "basic" scales, equivalent of the C and D scales, are 25 cm (10") long; on the other hand the inner tangent scale is only 12 cm (4.7 inches) long.
4. The rule has three parts. The "stock" with the scales marked in blue: the "slide" with the scale marked in red - they move in a circular grove in the stock and are fixed to a clear plastic disk: the cursor.
5. Another slight problem of this rule is that at the centre the cursor is raised slightly and, particularly with the trig scales near to the centre, it is difficult avoid parallax effects.

Front view
0c06-fearns-01.jpg (14472 bytes)
Detail - front view
0c06-fearns-02.jpg (30506 bytes)
Manufacturing date c 1970 - estimated
Length Circular, 13 cm, simplex
Material Plastic
Scales K, D, C, B, A, LL, S, T,
Cursor Plastic
Hair lines  
Documentation Original of leaflet
Condition Mint