A Dictionary of Music and Musicians/Tell-tale
TELL-TALE. A simple mechanical contrivance for giving information to an organ-blower (and sometimes also to an organist) as to the amount of wind contained in the bellows. A piece of string is fixed by one end to the top board of the bellows and carried over a pulley; a small metal weight is attached to the other end of the string. As the bellows rise the weight descends, as they sink the weight ascends; and the words 'Full' and 'Empty' mark the limits of the journey down and up.
[ J. S. ]