Patent ID: 7226713

Claim:
A carrier comprising: a magnetic core material; and a layer located on a surface of the magnetic core material, wherein the carrier satisfies the following relationships (1) to (3): 0.90≦(σ a/σb )<1.00 (1) 200≦(σ b·ρc )≦400 (2) 10≦(σ b/ρc )≦20 (3), wherein σb represents a magnetization (emu/g) of the carrier at 1,000 Oe, ρc represents a true specific gravity of the carrier, and σa represents a magnetization of the carrier determined by the following method including: (1) magnetically holding the carrier on a cylindrical sleeve having a magnetic pole area located over a magnetic pole and having a peak magnetic flux density of 100 mT in a direction perpendicular to an axis of the cylindrical sleeve; (2) rotating the cylindrical sleeve around the axis thereof for about 30 min; (3) removing the carrier from the magnetic pole area by applying a force which is three times as much as a weight of the carrier in the direction perpendicular to the axis of the cylindrical sleeve; and (4) measuring a magnetization at 1,000 Oe to determine the magnetization σa, wherein the carrier has a weight-average particle diameter (D 4 ) of about 25 to about 65 μm and includes carrier particles having a weight-average particle diameter not greater than about 12 μm in an amount not greater than about 0.3% by weight, wherein a ratio (D 4 /D 1 ) between the weight-average particle diameter (D 4 ) and a number-average particle diameter of the carrier (D 1 ) is about 1 to about 1.3, and wherein an electric resistance R is about is about 1.0×10 9 to about 1.0×10 11 Ω·cm when an AC voltage represented by the following formula (4) is applied at a frequency of 1,000 Hz to a magnetic brush of the carrier is formed between parallel plate electrodes having a gap of d mm such that magnetic brush has a space occupancy of about 40%: E ( V )=250 ×d (4), wherein d is 0.40±0.05 mm and E is a peak voltage.