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1. The central region about the longitudinal axis of an optical fiber, which region supports guiding of the optical signal. (188) Note 1: For the fiber to guide the optical signal, the refractive index of the core must be slightly higher than that of the cladding. Note 2: In different types of fibers, the core and core-cladding boundary function slightly differently in guiding the signal. Especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding. 2. A piece of ferromagnetic material, usually toroidal in shape, used as a component in a computer memory device. Note: The type of memory referred to has very limited application in today's computer environment. It has been largely replaced by semiconductor and other technologies. 3. The material at the center of an electromechanical relay or solenoid, about which the coil is wound. (188)