An apparatus for measuring the effect of an electric field on the viscosity of liquids has been developed, in which the liquid runs in a narrow channel between plane metal boundaries which can be used as electrodes. The apparatus allows the use of fields up to an average strength of 35 kV/cm. and gives a high degree of accuracy. With non-polar liquids, either dry or contaminated with water so as to conduct, and with non-conducting polar liquids an electric field transverse to the line of flow has no marked effect on the viscosity. With polar liquids that conduct relatively well, either normally or when contaminated with water, the electric field causes a considerable increase in apparent viscosity. This effect increases with increasing strength of field, but reaches a limiting value for very high fields. With polar liquids whose conductivity can be diminished by successive purifications the magnitude of this limiting effect is proportional to the conductivity of the specimen. With an alternating field the effect on viscosity remains constant, as frequency is increased, until a certain critical frequency is reached, and thereafter decreases with frequency, until it becomes too small to be detected. The critical frequency decreases as the distance between the electrodes increases and increases as the temperature is raised. There is no effect on viscosity with a field parallel to the lines of flow. All these experimental facts can be explained on the theory that the effect is due to the accumulation of ions that takes place in the neighbourhood of the electrodes when the liquid conducts. The ions act as centres round which polar molecules cluster. Thus for an effect to appear it is necessary to have a liquid which conducts and which is made up of polar molecules. The limiting increase of apparent viscosity which takes place at high fields is due to the fact that the increase of viscosity of the liquid near the electrodes slows down the ions, and thus tends to diminish the number of excess ions. The variation of the effect of an alternating field with frequency also finds a ready explanation in this theory. The theory has been made to account for various other observed facts.