A device used to trap ions by electrical or magnetic fields (or a combination of both). There are a number of types. The Paul trap has a ring electrode with a hyperbolic section and two hyperbolic end caps. Ions are trapped in an oscillating field produced by applying an oscillating voltage (about 1 MHz) between the ring electrode and the end caps. The device was invented in the 1950s by the German physicist Wolfgang Paul (1913–93). The Penning trap has a similar geometry and operation, but there is also a positive direct voltage (about 100 V) on the cap electrodes with respect to the ring electrode, and an axial magnetic field (about 5 tesla) to confine the particles. This type of trap was developed in 1959 by the German–American physicist Hans Dehmelt (1922–2017). He named it after Frans Penning, who had invented the Penning gauge, which also uses a magnetic field. Ion traps can be used for storing and investigating the properties of ions and other charged particles and can also be used in mass spectroscopy.