Magnetic developer, comprising spherical particles magnetic

Claim 1

A magnetic developer for developing electrostatic latent images, comprising hydrophobic silica fine powder and an insulating magnetic toner comprising at least a binder resin and a magnetic material; wherein 0.6 to 1.6 wt. parts of the hydrophobic silica fine powder is mixed with 100 wt. parts of the insulating magnetic toner; said magnetic developer having a BET specific surface area of 1.8 to 3.5 m 2 /g, a triboelectric chargeability of -20 to -35 μC/g, an aerated bulk density of 0.40 to 0.52 g/cm 3 , and a true density of 1.45 to 1.8 g/cm 3 ; said magnetic material having an average particle size of 0.1 to 0.35 micron and comprising at least 50% by number of spherical magnetic particles whose surfaces substantially comprise curved surfaces; said insulating magnetic toner containing 70-120 wt. parts of spherical magnetic particles with respect to 100 wt. parts of the binder resin; said developer containing 17-60% by number of magnetic toner particles having a particle size of no greater than 5 microns, containing 5-50% by number of magnetic toner particles having a particle size of 6.35-10.08 microns, and containing no greater than 2.0% by volume of magnetic toner particles having a particle size of at least 12.7 microns; wherein the magnetic toner has a volume-average particle size of 6-8 microns, and the magnetic toner particles having a particle size of no greater than 5 microns have a particle size distribution satisfying the following formula: N/V=-0.05N+k, wherein N is the percentage by number of magnetic toner particles having a particle size of no greater than 5 micron, V is the percentage by volume of magnetic toner particles having a particle size of no greater than 5 microns, k is a positive number of 4.6 to 6.7, and N is a positive number of 17 to 60.