Eddy current tests provide a very important testing technique for metals. The technique is based on the principle that when a coil carrying A.C. is brought near or made to surround a metallic specimen, eddy currents are induced in that component, the magnitude and distribution of which depend on the geometry of the system and the conductivity of the specimen.

The coil impedence is affected by the presence of eddy currents. The impedence change is resolvable to an inductive component, which depends on the space relationship between the coil and specimen, and a resistive component, depending on the conductivity of the specimen.

Conductivity is closely related to the metallurgical structure of the specimen and its state of heat treatment. Thus, a separation of these two effects gives a sensitive testing technique for the metals. The use of eddy currents for nondestructive testing was pioneered by F. Forster and his associates in Germany, and further development was made in Britain by E.G. Stanford and in the USA by R. Hoschild and others.