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Protractor: Plain Circular Brass Protractor with a Centre Bar (Stanley, 1878)
The draughtsman's circular protractor: a flat brass ring divided all round into degrees and half-degrees, its outer edge bevelled thin so a needle point can feel the divisions, with a bar across the middle whose edge runs exactly through the centre from 0° to 180°. Lay the bar on a line, prick off the angle and draw through the point and the centre.
Result: a protractor for setting off and measuring angles on a drawing.
Source: W. F. Stanley, A Descriptive Treatise on Mathematical Drawing Instruments (1878), pp. 227-228. Unknown: sizes and how the circle is divided. Picture: the book's figure.
Intermediate
Unknown
Instructions
1
1
Make the ring and bar
Make the ring and bar
Cut a flat ring of brass (electrum also serves) with a bar across the inner space. Make one side of the bar a true straight line through the centre of the ring, and mark the centre with a short line in the middle of the bar; a small semicircle may be cut out of the bar's edge there to make it easier to set over a point. Bevel the outer edge down very thin. Not given: sizes. Ours: brass sheet.
Materials for this step:
Brass Sheet1 ring and barTools needed:
Tin Snips
Metal Lathe
Unknown Dimensions2
2
Divide it
Divide it
Divide the circumference into degrees, subdivided into half-degrees, so that the bar's edge, carried on, reads 0° and 180°, and number the degrees. Not given: how the circle is divided.
Tools needed:
Unknown Dimensions3
3
Use it
Use it
To set off an angle from a point on a line, lay the bar so it half covers the line, the line reading 0 to 180, with the point in the centre. Prick off the number of degrees at the bevelled edge with a needle point, lift the protractor away, and draw a line through the mark and the point.
Tools needed:
Unknown DimensionsMaterials
1- 1 ring and barPlaceholder
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