Measurement Techniques

11.2.1 Four-Sensor Probe

The four-sensor probe [2] used in this study consists of a central front sensor and three peripheral rear sensors (Fig. 11.1a). Tungsten acupuncture needles with a maximum diameter of 0.1 mm were coated with epoxy resin varnish except the tip; the diameter of the tip is less than 1 pm. The insulated needles were inserted into a

a b

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Fig. 11.1 Schematics of four-sensor probe (a) and electromagnetic probe (b)

seven-bore insulating tube made of Al2O3. The output signals were acquired at a sampling frequency of 10 kHz and then processed on a PC.