An electrical instrument for measuring electric current. Currents are usually either unidirectional and steady (direct current or dc) or alternating in direction at a relatively low frequency (alternating current or ac). A current that is unidirectional but regularly fluctuating is a superposition of dc and ac. Higher-frequency ac is often referred to as radio-frequency or RF current. At frequencies above about 10 MHz, where the wavelength of the signal becomes comparable with the dimensions of the measuring instrument, current measurements become inaccurate and finally meaningless, since the value obtained depends on the position where the measurement is made. In these circumstances, power measurements are usually used.