| 摘要 |
A coil is provided to a developing tank containing a toner mixture consisting of ferromagnetic carrier particles and non-magnetic toner particles in such a manner as to be magnetically coupled with the mixture. The coil constitutes a frequency determining element of an oscillator. The inductance of the coil and thereby the frequency of the oscillator vary in accordance with the toner density or proportion of the toner particles in the mixture. An FM demodulator comprising a phase locked loop produces an output signal having a magnitude corresponding to the oscillator frequency and thereby the toner density. An electrically actuated valve is opened to supply more toner particles to the mixture when the toner density drops below a predetermined value.
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