PURPOSE: To decrease the dispersion of measurement sensitivity and facilitate setting, by causing a motor cap enclosing brushes and commutators to operate as an antenna.

CONSTITUTION: One end of an insulated signal wire 20 is connected to the steel-made motor cap 13 of a DC motor. This motor cap 13 encloses brushes 11 and commutator 12, is insulated electrically, and is insulated from the ground at the time of measurement. Accordingly, if spark discharge occurs between the brushes 11 and commutator 12 with the rotation of the motor 1, the motor cap 13 and a housing 15 connected to this electrically receive radiated electromagnetic waves. And a voltage amplifier 2 connected to the other end of the insulated signal wire 20 power-amplifies this voltage variation, and sends its output to an analog display. This prevents the positional change of the antenna, and diminishes the dispersion of the reception sensitivity because of it.