In set theory, a discipline within mathematics, the aleph numbers are a sequence of numbers used to represent the cardinality (or size) of infinite sets. They are named after the symbol used to denote them, the Hebrew letter aleph (\aleph). The cardinality of the natural numbers is \aleph_0 (read aleph-naught, aleph-null, or aleph-zero), the next larger cardinality is aleph-one \aleph_1, then \aleph_2 and so on. Continuing in this manner, it is possible to define a cardinal number \aleph_\alpha for every ordinal number α, as described below. The concept goes back to Georg Cantor, who d...

In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field. Displacement current has the units of electric current density, and it has an associated magnetic field just as actual currents do. However it is not an electric current of moving charges, but a time-varying electric field. In materials, there is also a contribution from the slight motion of charges bound in atoms, dielectric polarization. The idea was conceived by James Clerk Maxwell in his 1861 paper in connection with...