File:Lord Kelvin quadrant electrometer engraving.jpg

Summary
Scanned from the book "Elementary Lessons in Electricity and Magnetism" by Silvanus P. Thompson. 1905 Edition.

The moving sector, a very light aluminum foil disk, is suspended by a fine metal wire F to the terminal C where it is charged by an auxiliary charge. A small mirror M is attached to the bottom of the suspended sector. The four fixed sectors are directly connected to the four terminals 1,2,3 and 4, shown here connected diagonally (1 to 3 and 2 to 4). Potential difference applied between terminals 1 and 2 will cause a deflection of the moving sector and the mirror.