Developer density in an electrophotographic system is measured. The developer includes toner particles and magnetic carrier particles. First and second mutually-insulated electrodes define a working volume between them through which developer passes without contacting the first electrode. An AC voltage or current source is connected to the electrodes. An inductor is connected in series with a voltage source, or an inductor is provided in parallel with a current source. The voltage or current is applied and corresponding current or voltage across the electrodes is measured. The density of the developer in the working volume is automatically determined using a processor responsive to the measured current or voltage and the applied bias.