frequency of the oscillator is dependent upon the values of the inductive reactance of the coil l 1 and the capacitive reactance of c 1 the inductive reactance of the coil l 1 is varied by the proximity of armature 50 when current flows in coil l 1 eddy currents are generated in the armature plate 50 by virtue of the electromagnetic field generated by the current flow in coil l 1 the physical position of the armature 50 is changed by virtue of movement of diaphragm 18 in response to pressure changes in chamber 26 which in turn causes variations in the inductive reactance of coil l 1 in the present practice of the invention it has been found satisfactory to use a coil l 1 having an inductance of 2 3 millihenries with c 1 having a value of 0 01 microfarads when the voltage from the l c circuit to the collector of q 2 along lead 152 is high q 2 is off and q 1 is on when the voltage to the collector of q 2 is low q 2 is on and q 1 is off q 1 and q 2 thus act as an oscillator in conjunction with associated resistors r 2 r 5 r 4 r 6 and capacitor c 3 the oscillator circuit of the present invention has been found satisfactory if the values of the resistors r 1 r 4 r 6 and capacitor c 3 are chosen to give an oscillator frequency.