Klaverens Hus (the Keyboard House) in Söderhamn, north of Gävle, mentions psalmodikon at http://www.klaverenshus.se/Monochord.html. It defines it as a “variant of [the] monochord,” which seems accurate enough.
As the name indicates the monochord is an instrument equipped with one string stretched over a sounding-box. Its roots go down to antiquity when the mathematical interval relations with one another were discovered by using a monochord with a movable bridge that was placed in different positions of the string. Thus, monochords have been used as physical instruments to research and demonstrate the interval properties to our own times. The Swedish Academy of Science owned for example a monochord made here c. 1750 which was used at acoustical experiments. It was the time when the sound was researched physically and unequal, well-tempered systems and equal temperaments were much discussed and calculated. The function of ‘tuning aid’ was a closely related field of application where we today use electronic, computerized tuning devices.
A late, in Sweden wide spread variant of monochord is played with a bow. It was ‘invented’ by the rural dean Johannes Dillner (1785-1862) to improve the congregational hymn singing. Using a figure notation everybody could learn the hymn tunes at home and sing them at church.
The monochord in our collections is made by Frans Svanström & Cie. (KH 329), a well-reputed office and stationer’s shop that sold monochords for use in school physics rooms.
On the “Who we are” page in its website: “Klaverens Hus is a museum of such keyboard instruments for which we no longer have any domestic factories for uprights, square pianos and grand pianos (hammer instruments), harmoniums and reed organs and their respective self-playing variants. …”