Size for size Neodymium Magnets are the most powerful magnets to be developed also referred to as Earth Magnets. They may be used for many magnetic demonstrations where intense field strength is an important criterion. For example if a neodymium magnet is dropped down a length of copper plumbing pipe the eddy currents generated in the copper by the passage of the magnet create a retarding field great enough to measurably increase the time of fall of the magnet.Dimensions:4mm x 10mmPack of 10Alternative sizes are also available (B8A47144 - 20mm x 10mm B8A47132 - 10mm x 10mm)WARNING:Supervision required when using strong earth magnets.Harmful if swallowed.