The sector was invented or at least first deployed by Thomas Hood or Galileo Galilei at the turn of the 17th century. Although capable of many functions, you can use the sector to find proportions -- thus its other name, the proportional compass.

The phenomenon of the sector is a dramatic paradox. The sector was created to eliminate the need for tedious arithmetic, but its use accelerated the mathematics of natural science. According to *Wikipedia*, the sector advanced science itself.

The sector was a very useful instrument at a time when artisans and military men were poorly educated in mathematics and, often, were unable to perform even elementary arithmetical operations. The inaccuracy induced by the analog scales of the sector were usually of no concern to those attempting to find a rapid solution to an approximate problem. It is striking, however, that the disciplines to which these instruments were applied, particularly perspective, music, architecture and fortification, traditionally classed as mechanical sciences, soon emerged as mathematical sciences in the seventeenth century. Indeed there is evidence that the universality of these practical applications helped to make possible the universality of science at a theoretical level. Hence this technology was not simply a consequence of advances in science. Rather, the technology helped make possible the mathematical sciences that led to modern science.

As you can see from the fancy version, the sector serves many functions. You might find yours useful if not for fortification and gunnery practice, but as a good way to explore proportional equations. If you make a sector with 13 sections, you'll be able to approximate the golden ratio (1.618:1). Or in my case, as an American in London, I can use it as a quick currency or temperature conversion tool.

http://cogtoys.wordpress.com/2014/08/12/sectors/