A Maginnis’s Universal Sector in vulcanite with an adjustable German‑silver arm a drawing‑instrument with an electrum taper used for precise division of lines, arcs and angles, marketed to architects and engineers and dating from the late 19th century.
 
Measures 23 x 18cm.
 
A nice example in good condition, the Science Museum in London houses one in their collection which can be seen here f you are interested:
 
https://collection.sciencemuseumgroup.org.uk/objects/co60844/maginniss-universal-sector