For a person in a position of power:
* Monarch: King, queen, emperor, empress
* Sovereign: King, queen, supreme ruler
* Head of state: President, prime minister
* Leader: Chief, director, commander
* Governor: Administrator, director, superintendent
* Magistrate: Judge, justice of the peace
* Tyrant: Dictator, despot, autocrat
For a measuring tool:
* Straightedge: Measuring stick, yardstick
* Scale: Tape measure, measuring tape
* Measuring instrument: Caliper, protractor, level
Other related words:
* Administrator: Manager, director, supervisor
* Regent: Temporary ruler
* Viceroy: Governor of a colony
* Chieftain: Leader of a tribe or clan
* Autocrat: Absolute ruler
To give you the best synonym, it would be helpful to know what kind of ruler you're referring to.