Electrical frequency analyser

In this instrument the current to be analysed is caused to modulate the output from a variable frequency oscillator. The output thus modulated is passed to a vibrator unit similar to that of a Duddell oscillograph, but as sharply tuned as possible, the sharpness of tuning being increased by a resonant electrical circuit. The lowest frequency of the variable oscillator is made equal to the frequency of the tuned strip, and the frequency control of the oscillator is arranged to move a sensitive film so that the vibrations of the strip are automatically recorded.