This paper describes a medium-wave Adcock direction-finder capable of working over the wavelength range 750 to 2000 metres (frequency 400 to 150 kc.) with a pick-up factor between 3 and 5 metres. Under favourable conditions a bearing can be taken on a field strength of 10 microvolts per metre with a swing of less than 1°. When the field strength is 1 microvolt per metre the corresponding swing is ± 10°.The standard wave error is less than 1° as measured on a wavelength of 1 760 metres (frequency 170 kc), and this may most probably be taken to be representative of its value over the whole range. The instrumental error after balancing is less than 1°, but no information is at present available as to how long the balance remains constant.The instrument may therefore be said to constitute a practical development of the balanced-coupled Adcock system, retaining all the advantages of that type as regards polarization error but with a greatly enhanced pick-up factor compared with that of the early experimental model, which makes the apparatus capable of taking bearings with field strengths as low as 1 microvolt per metre, and with an instrumentalaccuracy as good as that of a loop-type Bellini–Tosi direction-finder.