The paper describes apparatus for obtaining for purposes of measurement a voltage proportional to, and in phase with, a given alternating current. The Introduction outlines defects in the water-cooled tube resistors which have been in use at the National Physical Laboratory for this purpose; these defects have led to the construction of air-cooled resistors for moderate currents, and current transformers with nickel-iron cores for heavy currents. Section (1) deals with the requirements for the air-cooled resistors, and with theoretical and practical points in their design; tests showing very satisfactory performance are described. Section (2) outlines the construction of the special current transformers, and deals at some length with the method of calibrating them. Finally, the effects of a number of variables on the transformer performance are investigated: the characteristics are shown to be very good, and no sign of instability is found.