A cup of iron, wood or some material that does not amalgamate or is
unattacked by mercury, which is filled with mercury and made an
electrode of a circuit. By dipping the other terminal of the circuit
into the mercury a very good contact is obtained. It is well to cover
the mercury with alcohol. The cup may be filled so that the mercury
rises in a meniscus or semi-globule above its edges.

For some purposes this form is useful, as for contacts with the end of a
swinging wire or pendulum, because in such cases the contact can be made
without the contact point entering the cup. The point swings through the
projecting meniscus without touching the edges of the cup. A mercury cup
and contact constitute a mercury break.