Abstract
A focusing system achromatic for the momentum deviations has been investigated. The system consists of a pair of alternate gradient electro-magnetic fields whose n-values, n, and nm, were designed ±5, respectively. Experiments were carried out using electrons with energies from 5 to 11 keV for various combinations of the electric field with the magnetic field. The momentum free condition was obtained when and only when the ratio of the electric force to the centrifugal force was equal to -1.