sentences:  Differences in magnetic coil design should be considered when comparing results, with important elements including the type of material, geometry and specific characteristics of the associated magnetic pulse.The core material may be either a magnetically inert substrate ('air core'), or a solid, ferromagnetically active material ('solid core').Solid cores result in more efficient transfer of electrical energy to a magnetic field and reduce energy loss to heat, and so can be operated with the higher volume of therapy protocols without interruption due to overheating.