says that a particle of charge in an electric field experiences an electric force: The second term is the magnetic force: Using the definition of the cross product, the magnetic force can also be written as a scalar equation: where , , and are the scalar magnitude of their respective vectors, and is the angle between the velocity of the particle and the magnetic field. The vector is defined as the vector field necessary to make the Lorentz force law correctly describe the motion of a charged particle. In other words, The field can also be defined by the torque on a magnetic dipole, . The SI unit of is tesla (symbol: T). The Gaussian-cgs unit of is the gauss (symbol: G). (The conversion is 1 T ≘ 10000 G.) One nanotesla corresponds to 1 gamma (symbol: γ). The H-field The magnetic field is defined: Where is the vacuum permeability, and is the magnetization vector. In a vacuum, and are proportional to each other. Inside a material they are different (see H and B inside and outside magnetic materials). The SI unit of the -field is the ampere per metre (A/m), and the CGS unit is the oersted (Oe). Measurement An instrument used to measure the local magnetic field is known as a magnetometer. Important classes of magnetometers include using induction magnetometers (or search-coil magnetometers) which measure only varying magnetic fields, rotating coil magnetometers, Hall effect magnetometers, NMR magnetometers, SQUID magnetometers, and fluxgate magnetometers. The magnetic fields of distant astronomical objects are measured through their effects on local charged particles. For instance, electrons spiraling around a field line produce synchrotron radiation that is detectable in radio waves. The finest precision for a magnetic field measurement was attained by Gravity Probe B at (). Visualization The field can be visualized by a set of magnetic field lines, that follow the direction of the field at each point. The lines can be constructed by measuring the strength and direction of the magnetic field at a large number of points (or at every point in space). Then, mark each location with an arrow (called a vector) pointing in the direction of the local magnetic field with its magnitude proportional to the strength of the magnetic field. Connecting these arrows then forms a set of magnetic field lines. The direction of the magnetic field at any point is parallel to the direction of nearby