So … better late, I guess, than never:
A psalmodikon (pron. sahl-MOWD-i-kon) was a monochord, or one-string box fiddle, used by Scandinavian church musicians to help keep singers on pitch when they were learning new hymns. They were used primarily for choir practice and home services (husandakter) in the early to mid-1800s, falling out of use as more congregations were able to afford pump organs for worship. In Sweden, they were influenced by the hummel -- a box zither similar to a mountain dulcimer -- and they are fretted diatonically like a northern European hummel or an American dulcimer. They were brought to the U.S. by Swedish and Norwegian immigrants.
Here's what one looks -- and sounds -- like. The still picture below illustrates a YouTube audio clip of a psalmodikon ensemble in Stockholm playing an old Swedish hymn called "Din klara sol går åter pop" [the glorious sun doth arise]:
Explains YouTube user Martin Magnusson (in Swedish, followed by English translation): "Din klara sol går åter opp ... ett smakprov från Nordiska psalmodikonförbundets CD-skiva Psalmer och visor på psalmodikon inspelad i Stjärnhov i juli 2010. Kan beställas från NPsF hemsida www.npsalmodikonforbundet.se." [The Glorious Sun Doth Arise … a sample from the Nordiska psalmodikonförbundet's CD Hymns and songs on the psalmodikon played in Stjärnhov, July 2010.]
In recent years, the instrument has been revived by primarily Norwegian-American groups in the St. Paul-Minneapolis area, who are members of the Nordic-American Psalmodikonforbundet, and in Sweden by the Nordiska Psalmodikonförbundet. Their websites are linked below.
Notation. One of the psalmodikon's selling points was a system of tablature called "siffernoter" [numerical notation] that substituted the numbers for different degrees of the scale -- the second, the third, the fourth, the fifth and so on -- in place of the notes on the lines of a musical staff like in standard notation. "Din klara sol …" looks like this.
And here, for the sake of comparison, is "Din klara sol …" in standard notation, as seen in the Augustana Synod's 1892 edition of the 1819 Svenska Psalmbok:
They've transposed it, from E-flat in Dillner's siffernoter to D in the 1892 hymnal. But the tablature doesn't change -- to change keys on a psalmodikon, you would just retune the melody string from Eb to D. I am betting that would have been very useful for pastors in little churches out on the prairie who might have to accommodate the singers in a small choir from time to time.
* I should add: I would definitely change one of Wikipedia's "basics." They spell "psalmodikon" with a "c," but the word is spelled with a "k" in English, Norwegian and Swedish alike. The "p" at the beginning of the word is optional in Norwegian, for reasons too complicated to go into here, but nobody since the 1850s has spelled the name of the instrument with a "c." Nobody.