Historical clue formulations

• Shape of the heart's electromagnetic field — 2023-04-22
• Shape of a doughnut — 2021-09-19
• Something ring-shaped — 2017-05-26
• Hula hoop, for one — 2015-05-09
• Doughnut's shape — 2014-06-24
• Life preserver, e.g. — 2009-10-18
• Geometrical solid — 2002-05-02
• Anchor ring — 2001-11-10
• Doughnut's shape — 1998-05-14
• Doughnut mathematically — 1996-02-06
• Topological solid — 1995-09-23
• Doughnut, e.g. — 1994-09-28