An antenna in the form of a slot in a metal screen. As a rule the slot is cut across the lines of the currents induced in a continuous screen by an electromagnetic field (or at a sufficiently large angle to them). This distorts the distribution of the current in the screen and produces an alternating voltage (an alternating electric field) across the slot, and it is this that gives rise to the radiation. The radiation can be found as that of a certain fictitious "magnetic current" flowing along the slot in the screen. The slot antenna is therefore called magnetic. Slot antennas are often used to excite waveguides and cavity resonators, and also as primary feeds in complex antennas.