PURPOSE: To enable the selective collection of the discharge information of the indivisual coil constituting winding without disassembling the electric connection of the winding, by providing an electromagnetic probe, a comparator, an integrator and a monostable multivibrator.

CONSTITUTION: An electromagnetic probe 1 is formed by winding a magnetic wire 1b around a circular arc shaped ferrite core 1a and the output from the wire 1b is inputted to a preamplifier 3 through a coaxial cable 2 of which the outside is earthed to be converted to a pulse signal with proper magnitude. This signal is detected and rectified by a detection rectifier 5 through an automatic gain control amplifier 4 and compared with reference voltage 10 by a comparator 6. When the pulse signal is larger as the result of comparison, a monostable multivibrator 7 is driven and stepping voltage is applied to an integrator 8. The output of the integrator 8 is subjected to negative feedback as the control signal of the amplifier 4 and indicated by an indicator 9. This indicated value is proportional to the max. value of a steadily detected corona pulse and the discharge information (corona discharge intensity) of an indivisual coil can be detected within a short time.