A test used to determine the impact toughness of a piece of material, usually a metal.
The machine used in the test is essentially a large, rigid, heavy pendulum attached to a rigid base. The pendulum is raised up to a predetermined height, where it clicks into place. The pendulum therefore has a known amount of potential energy before it is released. The test specimen is secured at the bottom of the pendulum's swing so that the kinetic energy of the pendulum will be known at the point of impact. The standard test piece is a cuboid measuring 10mm x 10mm x 55mm with a standard V-shaped notch cut out of it halfway along its length. A button is pressed to release the pendulum which swings down and smacks into the test piece, generally sending half the sample flying across your lab. As the pendulum swings, it moves a small, light pointer which sticks at the top of the pendulum's stroke, recording the extent of its movement. The energy absorbed during the impact can then be read off a scale on the machine - no electronics necessary. The amount of energy absorbed by the plastic deformation of the sample tells you its toughness (more energy absorbed means higher toughness).
This can be useful for quality control purposes to ensure that a given batch of metal reaches a minimum toughness standard. By running a series of tests, it can also be used for determining the change in a material's toughness over a range of temperatures, known as the brittle to ductile transition. An analysis of material recovered in 1985 showed that the toughness of the steel used in the construction of the Titanic decreased significantly when the metal was cooled down to -1oC in the cold waters of the Atlantic. To what extent this embrittlement contributed to the fate of the Titanic is unknown. It is well-known that a combination of poor design and embrittlement at low temperatures caused the loss of numerous Liberty Ships in the 1940s.
Since the Charpy Test was first devised, the machines have been refined and high-tech electronic versions produced. Furthermore, a whole new area of science studying these sorts of things has sprung up. In modern metallurgy, therefore, the use of the Charpy Test (and the pretty similar Izod Test) is often supplemented by established scientific theory and a bunch of nasty formulae.
You can find some videos of Charpy Tests in action here: