Laboratory measurements were made of the instabilities of the Ekman layer using hot wire anemometers. The apparatus consisted of two parallel circular rotating plates forming a spool; the air was admitted through screens at the outer edge and removed through a screen cage at the hub. In the Ekman layers formed on the inner surfaces of the plates, measurements were made of the mean velocities as functions of r and a; the velocity fluctuations were also measured.
It appears that the instability labelled type II by Faller always occurs first, and at zero Rossby number the critical Reynolds number is 56 ± 2. This instability originates in the boundary layer, but at slightly higher Reynolds number the fluctuations persist far into the geostrophic region, probably as inertial waves excited by the boundary-layer fluctuations.
At higher Reynolds number another instability appears of shorter wavelength and slower speed. This instability is confined to the boundary layer and is apparently the type I reported by Faller.
The phase speeds, frequencies, and wave-front orientations of both type instabilities have been measured.