This paper contains an account of the advances in design and theory of the screened hot-wire ammeters described in 1930. The design has been improved both mechanically and electrically, leading to more compact instruments of lower impedance. These improvements have arisen from a more careful consideration of the temperature distribution in the hot wire, the behaviour of the magnifying system, and the corrections necessary at high frequencies. The results of experience with a number of experimental instruments are given and constitute a basis for any further designs that may be required.