Lippmann electrometer

A Lippmann electrometer is a device for detecting small rushes of electric current. It was invented by Gabriel Lippmann. The device consists of a U-tube which is thick on one end and very thin on the other. The thin end is designed to act as a capillary tube. The u-tube is half filled with mercury with a small amount of dilute sulphuric acid above the mercury in the capillary tube. Metal wires are connected at the fat end into the mercury and at the thin end into the sulphuric acid.

When the pulse of electricity arrives it changes the surface tension of the mercury and allows it to leap up a short distance in the capillary tube. This device was used in the first practical ECG machine which was invented by Augustus Desiré Waller.