JPS6339859B

PURPOSE:To positively perform the detection of a soft spot from the transformation ratio, by a method wherein an ultrasonic wave signal is transmitted, while a load is being applied to the layer of an insulated tape in its radial direction, to measure the difference between the time when the signal is transmitted and that when a reflected wave is received and to obtain the change of the thickness of the insulated tape. CONSTITUTION:When an ultrasonic wave P1 in the form of a pulse is transmitted in the radial direction of an ultrasonic wave tranceiver 4 placed on the surface of the layer of an insulated tape 3, the ultrasonic wave P1 is allowed to penetrate through the insulated tape layer within the time corresponding to the propagation constant of the layer. The wave having reached a conductor 2 is reflected from its surface. The reflected wave P2 is received by the ultrasonic wave tranceiver 4 after it has penetrated through the insulated tape layer 3. For this reason, the wave does not affect the insulated tape layer 3 located between the conductor 2 and a supporting board 5. Therefore, by measuring the time DELTAt between the transmission and reception of the ultrasonic wave pulse as shown in the figure, or the delayed time of the ultrasonic wave receiving pulse, it is possible to reliably detect only the thickness of the insulated tape layer between the ultrasonic pulse transmitting surface and the conductor.