There is a similarity between an electret and the dielectric layer used in capacitors ; the difference is that dielectrics in capacitors have an induced polarisation that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or polarisation. Some materials also display ferroelectricity (i.e. they react to the external fields with a hysteresis of the polarisation). Ferroelectrics can retain the polarisation permanently because they are in thermodynamic equilibrium, and thus are used in ferroelectric capacitors. Although electrets are only in a metastable state, those fashioned from very low leakage materials can retain excess charge or polarisation for many years. An electret microphone is a type of condenser microphone that eliminates the need for a polarisation voltage from the power supply by using a permanently charged material.