The first mention of a harpsichord was by a Jurist in Padua who wrote In 1397 that Hermann Poll claimed to have invented an instrument called the 'clavicembalum' . The harpsichord Is a stringed keyboard instrument in which the strings are plucked by a crow-quill plectrum mounted on the end of the key. It is characterized by an elongated wing shape like that of a grand piano, but with the strings running directly away from the player, (the grand piano of today has strings that run diagonally from the player). Other smaller harpsichord-like instruments include the virginal with its transverse stringing, and the spinet with oblique stringing. Although popular for centuries, they had one drawback , the harpsichords could not make changes in dynamic expression with changes in the player's touch. British Harpsichord Society
The clavichord has a rectangular shaped body. The string is struck by a metal tangent mounted on the end of the key. The loudness of the tone depends on how hard the key is depressed. The string continues to vibrate while the tangent is in contact with the string. The player, therefore, influences the tone of the string with variations of pressure on the key. Bach is reported to have preferred the clavichord as the more expressive of the keyboard string instruments. However, the clavichord was small and not loud enough to accompany other instruments. Also, It is not easy to play well because the finger pressure on the key changes the tone, pitch, and dynamic level.
Both the clavichord and the harpsichord were very popular from the 15th through the 16th Centuries. After about 1810 the more powerful and versatile piano almost completely replaced both of them . Their use was revived in the late 1880's by Arnold Dolmetsch . Some manufacturers make them today. Especially popular are the make-it-yourself kits.
Quoted from Zuckermann Harpsichords International : The modern harpsichord derives from several historical periods and a number of countries; nevertheless, every harpsichord or virginal works the same way. When a key is pressed, its far end rises a slip of wood called a jack. The jack is fitted with a plectrum of bird feather or plastic, which plucks the string. The string vibrates and transmits its energy to the soundboard, which vibrates in response, creating pressure waves in the air around it. These waves are transmitted to your ears, and through a complex mechanical and neurological process your brain converts that energy into the incomparable glittering, majestic sound of the harpsichord.
Resources. The Harpsichord Clearing House is the most comprehensive resource for all early keyboard needs--be it harpsichord, virginal, spinet, clavichord, fortepiano or continuo organ-- in North America. http://www.harpsichord.com/
Instructions for building a harpsichord: http://www.geocities.com/Vienna/Studio/1714/italian.html