25 mm (about .98″) by 8 mm Teflon Magnetic Stir Bar.

Use this in combination with an Erlenmeyer Flask and Stir Plate to make a yeast starter.  A stir plate… keeps yeast in constant contact with nutrients, encourages CO2 to off gas and continuously incorporates oxygen all of that means higher cell counts.

Bel-Art F37120-0025 Spinbar Teflon Polygon Magnetic Stirring Bar; 25 x 8mm, White, without Pivot Ring


Also Consider: StirStarter Stir Plate | Yeast Starters and Fermentation Resources