The power transferred increases with frequency and the mutual inductance formula_1 between the coils, which depends on their geometry and the distance formula_2 between them. A widely used figure of merit is the coupling coefficient formula_3. This dimensionless parameter is equal to the fraction of magnetic flux through the transmitter coil formula_4 that passes through the receiver coil formula_5 when L2 is open circuited. If the two coils are on the same axis and close together so all the magnetic flux from formula_4 passes through formula_5, formula_8 and the link efficiency approaches 100%. The greater the separation between the coils, the more of the magnetic field from the first coil misses the second, and the lower formula_9 and the link efficiency are, approaching zero at large separations. The link efficiency and power transferred is roughly proportional to formula_10. In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameter formula_11, usually within centimeters, with the coils' axes aligned. Wide, flat coil shapes are usually used, to increase coupling. Ferrite "flux confinement" cores can confine the magnetic fields, improving coupling and reducing interference to nearby electronics, but they are heavy and bulky so small wireless devices often use air-core coils.