Didn't hold up long and generated a lot of heat. Core - small metal rings. Magnets tip a ring to left or right, which represents on and off. Semiconductor - integrated circuit on a chip. This is what modern computers use for memory.
Smaller numbers are better since that means it takes less time to access one memory address.
The capacity of a memory chip is measured in megabytes . Motherboards have only so many slots for memory so there are limits. Some motherboards require that all slots be filled and that all slots contain the same size memory board.