A Dictionary of Music and Musicians/Jack
JACK (Fr. Sautereau; Ital. Saltarello; Ger. Docke, Springer). In the action of the harpsichord tribe of instruments the jack represents the Plectrum. It is usually made of pear-tree, rests on the back end of the key-lever, and has a moveable tongue of holly working on a centre, and kept in its place by a bristle spring. A thorn or spike of crowquill projects at right angles from the tongue. On the key being depressed the jack is forced upwards, and the quill is brought to the string, which it twangs in passing. The string is damped by the piece of cloth above the tongue. When the key returns to its level, the jack follows it and descends; and the quill then passes the string without resistance or noise. In some instruments a piece of hard leather is used instead of the quill. In cutting the quill or leather great attention is paid to the gradation of elasticity which secures equality of tone. A row of jacks is maintained in perpendicular position by a rack; and in harpsichords or clavecins which have more than one register, the racks are moved to or away from the strings by means of stops adjusted by the hand; a second rack then enclosing the lower part of the jack to secure its position upon the key. We have in the jack a very different means of producing tone to the tangent of the clavichord or the hammer of the pianoforte. The jack, in principle, is the plectrum of the psaltery, adjusted to a key, as the tangent represents the bridge of the monochord and the pianoforte hammer the hammer of the dulcimer. We do not exactly know when jack or tangent were introduced, but have no reason to think that the invention of either was earlier in date than the 14th century. By the middle of the 16th century the use of the clavecin instruments with jacks had become general in England, the Netherlands and France; and in Italy from whence they would seem to have travelled. They were used also in Germany, but the clavichord with its tangents asserted at least equal rights, and endured there until Beethoven. The first years of the 18th century had witnessed in Florence the invention of the hammer-clavier, the pianoforte; before the century was quite out the jack had everywhere ceded to the hammer. Although leather for the tongue of the jack has been claimed to have been the invention of Pascal Taskin of Paris in the 18th century (his much-talked-of 'peau de buffle'), it has been found in instruments of the 16th and 17th; and it may be that leather preceded the quill, the introduction of which Scaliger(1484–1550) enables us to nearly date. He says (Poetices, lib. i. cap. lxiii) that when he was a boy the names clavicymbal and harpsichord had been appellations of the instrument vulgarly known as monochord, but that subsequently points of crowquill had been added, from which points the same instrument had become known as spinet—possibly from the Latin 'spina,' a thorn, though another and no less probable derivation of the name will be found under Spinet.
Shakspeare's reference to the jack in one of his Sonnets is well-known and often quoted—
'Do I envy those jacks that nimble leap
To kiss the tender inward of thy hand';
[ A. J. H. ]