Altogether, 25 terpodions were built, most of them by Friedrich. His brother Eduard worked mainly on the wooden cases and inlays, and was in charge of the appearance of the instruments, though the brothers lived and worked in different German cities. Nearly all the terpodions ever built are still in existence in different European museums.
Johann and Eduard meanwhile continued their advertising journeys throughout Europe. Besides their performances, they had to service the instruments they had already sold, as they did not function very reliably over a longer period. This may well have been a principal reason why Friedrich was looking for other methods of reliable sound generation for tuning purposes. It is certain that they must have become aware of all kinds of modern developments in this area as they travelled through different countries, which contributed to Friedrich's further refinement of the physharmonica.
In Berlin in 1828 Friedrich built an instrument, originally intended only for use as a tuning aid, which at first consisted of 21 different metal free reeds fastened to a wooden block in such a way that it was possible to blow the reeds individually. He then experimented further by fastening the reeds inside a small box "4 inches square and equally high" (in other words a cubic box with 4" sides), each of which could be made to vibrate by blowing through 21 individual "tone chambers" (Tonlochkanzellen). This instrument he named an aeoline.
The earliest experiments with the aeoline may have taken place in 1824, when it has been claimed that Buschmann built a tuning aid named an aura, about 4 inches long and equipped with 15 reed tongues. (The name Aura was also then in use in German to mean a Jew's harp).
In Buschmann's letters to his uncle, brother, and father it appears that he built a bigger version of an aeoline in 1829, with bellows and piano keyboard, which, being about the size of a small writing desk, was still much smaller than any comparable fixed key instrument they had built previously.
The Buschmanns certainly knew of an instrument built at about this time by Johann Caspar Schlimbach, an instrument maker trained in Vienna, and his cousin Bernhard Eschenbach in Königshofen in Bavaria: this was a pianoforte with an aeoline register. Schlimbach made no attempt to protect his invention, but freely showed the instrument to everyone who wanted to see it, with the inevitable result that a number of people patented very similar instruments in Vienna. Indeed, Buschmann's father Johann wrote in a letter of 30 October 1829 that he was thinking of taking out a patent for the new instrument in Bavaria.
Nor, in his impressively well-documented family history, was Buschmann able to confirm the story (see Sources).