This is the classic device used by Coulomb for the study that led to the famous law that bears his name. The thread, by which the glass rod with the metal disc was attached, is missing.
One of the charges is placed on the small stationary conducting sphere on the lower end of the vertical insulating rod. The other charge (of the same sign) is on a second small conducting sphere placed on the end of the counterbalanced crossbar, suspended by a torsion fiber. The top of the fiber is attached to a rotating cap, marked off in degrees. A paper scale pasted around the outside of the lower glass cylinder indicates the angular position of the rotating system.
Two tags indicate “142” and “144”.
Other torsion electrometers made by Pixii may be seen at: