Museo Galileo
Virtual Museum
Protractor (goniometer)
Protractor (goniometer)

Generic name for instruments used to divide circles and measure plane angles. They can be circular, semicircular or rectangular. To increase its accuracy, the protractor was often inserted in a nonius, a device for enhancing the precision of the measurements made by a direct readout on a graduated scale. The instrument often came with other accessories, such as folding metal arms and plumb lines.