Half-Helmholtz coils and a new sectorial coil scheme

When Helmholtz coils produce a uniform field, access to the uniform region may be hindered for vision or apparatus by one of the two coils. “Half-Helmholtz coils” have been devised to remove this hindrance. By using opposed compensating coils, nearly all of the coil bulk is moved to one side of the operating plane. Also described is a new sectorial coil idea that should greatly facilitate the experimental approach to magnetic lens design when such compensating coils are used.