| 摘要 |
A device for testing and/or measuring electrically conductive material comprises means 1 for generating at least two frequencies or frequency components omega 1- omega n which via a transmitter 2 induce a magnetic field in the material, and a control servo 5 for compensation of signals of the frequencies or frequency components derived from the transmitter 2. The compensation is such that an error signal resulting from a varying parameter such as varying lift-off, temperature or dimension is generally suppressed. This can be achieved by suppression of relatively slow signal variations but not the more rapid signal variations arising from the changes or deviations to be measured. <IMAGE> |
