provides the test pattern signal in which the carrier frequency pattern is modulated by the selected signal frequency pattern the frequency spectrum of the test patterns reaches from 3 hz to 14 khz in fig1 there is provided a separate output ep out 18 which makes the test signal available to external probes using test chamber 20 of the apparatus the test pattern signal is supplied to a primary excitation coil 22 which generates the electro magnetic test field the matter to be test 24 is placed inside coil 22 a secondary coil 26 senses the resultant electromagnetic field inside test chamber 20 the electrical output signal of secondary coil 26 includes a component from the test pattern field and a component representing the resonance signal of the matter under test responding to the test field a low pass filter 28 eliminates all frequency components above a certain frequency the resultant filtered signal is compared with the electrical test pattern signal in comparator 30 the difference signal is amplified in amplifier 36 and supplied to an electronic switch 38 switch 38 is provided to select either an external signal ep in from an external device connected to line 34 or the filtered differential signal from amplifier 36 switch 38 is controlled by a select signal on line 40 the selected signal is amplified in amplifier 42 and provided to two evaluation channels the first evaluation channel converts the received signal into a digital signal in d a converter 44 the resultant digital signal can be evaluated processed stored modified or treated otherwise in an processing device 46 such as a special or common purpose computer the second evaluation channel includes a demodulator 56 a voltage controlled oscillator 58 and an acoustic output device such as a loudspeaker.