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.

中级

Unknown

说明

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.

此步骤所需材料：

黄铜板1 ring and bar

所需工具：

铁皮剪

金属车床

未知尺寸

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.

所需工具：

未知尺寸

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.

所需工具：

未知尺寸