The compass is made up of two rods, wooden or metal, usually of equal length, articulated in the upper part through a simple system with two toothed wheels. At the base of the two rods, sometimes extendable, you can find different tools, depending on the function reserved for the compass: they are in any case constantly constituted by a fixing system (needle or suction cup) and by a writing device (graphite lead, chalk, marker or ink pen). In some compasses, especially professional ones, there is the possibility of changing the writing tool according to the chosen support material (blackboard or different types of paper and cardboard).
Based on the above characteristics, the compasses are divided into:
helix precision thumbwheel compass
(like the one in the figure), if they have a needle as a fixing tool, a mine or a nib as a writing system and a wheel in the center for adjusting the opening;
, when they have the same peculiarities as balustrades but lack a regulating wheel (the compass is kept in the chosen angle only thanks to the hardness of the junction gears between the rods, which must therefore be regularly tightened);
, if they have a suction cup as a fixing tool and a chalk or blackboard marker as a writing system; typically, they are made of wood or aluminum-based metal alloys (to make them lighter due to their large size).
The perfect compass
(The so-called perfect compass).
All the compasses we talked about above are useful, but they allow you to directly draw only their circumferences or arcs; on the contrary, the so-called perfect compass allows you to create images even of hyperbolas, parabolas and ellipses. It is composed, like all the other compasses, by two rods hinged by means of a gear;
the main difference, which then determines the different functioning, consists in the fact that one of the two rods can be lengthened, increasing the eccentricity of the figure. An operating diagram of the perfect compass is shown in the figure on the side: in it you can easily see how the drawing tool in question allows you to generate the image of an ellipse by intersecting a virtual cone on the α plane (which can consist of a sheet or any other graphic support). Precisely on the basis of the principle of intersection of the virtual cone it is easy to understand how this instrument is able to represent all conics.
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The content of this page was taken from: https://en.wikipedia.org/wiki/Compass_(drawing_tool)