Early 1800s

[kwired]

It is for measuring charge. It has two metal leafs which separate when the charge is passed to them.

I remember having one of those in Jr high science classes, and doing experiments with making static charges and touching an object to it - if the object were charged that charge was passed on to the plates and they would separate. Could normally discharge it just by touching the "input rod" with your bare hand.

Seems that is just one type: https://en.wikipedia.org/wiki/Electrometer