In studies of engines and engine lubricants, it is often desirable to measure clearances between shafts and sleeve—or journal-type bearings during operation. However, it is usually rather difficult to obtain such measurements without affecting the operation, particularly at high speeds. A method recently developed by M. L. Greenough and associates of the National Bureau of Standards for the Navy Bureau of Ships appears to offer a satisfactory solution of the problem. The heart of the new system is a mutual-inductance type of electrical distance-measuring element; variation of the distance of the rotating shaft from two small fixed coils results in a readily measurable variation in the coupling between the coils.