techniphone

from The Century Dictionary.

• noun A soundless apparatus for the gymnastic training of the hands of organists and pianists, and for the acquirement of a strictly legato touch.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Music) A dumb gymnastic apparatus for training the hands of pianists and organists, as to a legato touch.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device used to train the hand muscles used in playing the piano, in order to acquire advanced techniques such as legato