Many of our spare high frequency diaphragms will come supplied with a syringe of Ferrofluid. Ferrofluid is used to allow the voice coil of your HF unit to move freely within the magnet without overheating. It is not essential that you use the fluid provided, it is there if you need it. Many tweeters and HF diaphragms can burn out if overdriven and this will cause the fluid to evaporate. If your magnet gap does not contain any fluid then please use the syringe provided to top this up. If you have any further queries regarding ferrofluid then please submit a query to a member of our support team.