Definition of proof plane in US English:

proof plane


  • 1Electricity a small flat conductor with an insulating handle that is used to measure the electrical charge of a body by picking up the charge when placed in contact with the body and transferring it to a measuring instrument.

  • 2An object with an optically flat surface; a surface for use as a standard of flatness.


Mid 19th century; earliest use found in Philosophical Transactions of the Royal Society of London.