Make Unknown
Model Sector

1. The sector was a forerunner of the slide rule and used the principles of similar triangles to perform calculations with the aid of a pair of dividers.
2. If you look carefully at the brass pins beside the words tan, sin and num(ber) you can see that they they have a small hole to take the point of the dividers.
3. Chris Sangwin describes an identical rule on his site and gives more information its use.
4. I also show images of another sector. This one is in better condition but looks to be more modern

Front view
0008-sector01.jpg (27940 bytes)

Labelling of Chords and Tangents
0008-sector03.jpg (32381 bytes)

Labelling of sine and chord

Scale of sines and tangents
The values of sin 30 (=0.5) and tan 45 (=1.0) can clearly be seen.
0008-sector02.jpg (27130 bytes)
The small area of raised laminated ivory can be see to the left.

Rear view
0008-sector04.jpg (27827 bytes)

Labelling of Secants is clear but for other lines is less so.
0008-sector05.jpg (38335 bytes)

Labelling of sine and tangent

Manufacturing date At the moment I am placing it the first rule at early 19th century but I suspect it may be earlier. It is certainly hand engraved and the lettering is of a similar style to my Coggeshall rule dated to 1820. There is no clear indication of maker. The second rule is later, possibly second half of the 19 th. century.
Length 16 cm by 3.8 cm
Material Laminated ivory and brass.
Scales None
Gauge points None
Cursor None
Hair lines None
Documentation Instructions for a typical sector can be found here.
Condition The ivory is discoloured and in a few place darkened and pitted. In one place the lamination is lifting up. The hinge moves smoothly.