the magnetic field experiences Lorentz force. In a microscopic view, the Lorentz force is applied on the electrons in the eddy current. In a macroscopic view, the Lorentz force is applied on the surface region of the material due to the interaction between electrons and atoms. The distribution of Lorentz force is primarily controlled by the design of magnet and design of the electric coil, and is affected by the properties of the test material, the relative position between the transducer and the test part, and the excitation signal for the transducer. The spatial distribution of the Lorentz force determines the precise nature of the elastic disturbances and how they propagate from the source. A majority of successful EMAT applications are based on the Lorentz force mechanism. Magnetostriction A ferromagnetic material will have a dimensional change when an external magnetic field is applied. This effect is called magnetostriction. The flux field of a magnet expands or collapses depending on the arrangement of ferromagnetic material having inducing voltage in a coil and the amount of change is affected by the magnitude and direction of the field. The AC current in the electric coil induces an AC magnetic field and thus produces magnetostriction at ultrasonic frequency in the material. The disturbances caused by magnetostriction then propagate in the material as an ultrasound wave. In polycrystalline material, the magnetostriction response is very complicated. It is affected by the direction of the bias field, the direction of the field from the AC electric coil, the strength of the bias field, and the amplitude of the AC current. In some cases, one or two peak response may be observed with the increase of bias field. In some cases, the response can be improved significantly with the change of relative direction between the bias magnetic field and the AC magnetic field. Quantitatively, the magnetostriction may be described in a similar mathematical format as piezoelectric constants. Empirically, a lot of experience is needed to fully understand the magnetostriction phenomenon. The magnetostriction effect has been used to generate both SH-type and Lamb type waves in steel products. Recently, due to the stronger magnetostriction effect in nickel than steel, magnetostriction sensors using nickel patches have been developed for the nondestructive testing of steel products. Comparison with piezoelectric transducers As an ultrasonic testing (UT) method, EMAT has all the advantages of UT compared to other NDT methods. Just like piezoelectric