The dynamic method of determining thermal diffusivities devised by King has been improved. A sinusoidal temperature is impressed on one end of a wire specimen, and the thermal diffusivity is determined from the measured decrement of the temperature wave traveling along the specimen. Elimination or determination of heat losses is unnecessary. Greater precision is possible with this method than with previous methods. The thermal diffusivity of a pure specimen of nickel, measured at 25°C, was 0.15885 cm2/sec., with a probable error of 0.06 percent. The thermal conductivity of nickel corresponding to this determination is 0.618 watts/cm °C.