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An apparatus for monitoring a device (such as a solenoid) powered by the apparatus includes a power supply for providing an alternating power signal to the device and a monitoring circuit that measures a prescribed characteristic of the alternating power signal. A control circuit is responsive to me

An apparatus for monitoring a device (such as a solenoid) powered by the apparatus includes a power supply for providing an alternating power signal to the device and a monitoring circuit that measures a prescribed characteristic of the alternating power signal. A control circuit is responsive to measurements from the monitoring circuit and is arranged to analyze the measurements to determine the status of the device therefrom.

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An apparatus for monitoring a device (such as a solenoid) powered by the apparatus includes a power supply for providing an alternating power signal to the device and a monitoring circuit that measures a prescribed characteristic of the alternating power signal. A control circuit is responsive to me

An apparatus for monitoring a device (such as a solenoid) powered by the apparatus includes a power supply for providing an alternating power signal to the device and a monitoring circuit that measures a prescribed characteristic of the alternating power signal. A control circuit is responsive to measurements from the monitoring circuit and is arranged to analyze the measurements to determine the status of the device therefrom. generator for applying an electrical current between two of said injection electrodes, said current generator delivering alternating current at a frequency lying in the range 5 kHz to 100 kHz and at a voltage of less than 1000 volts; at least one support cable provided with electrical conductors, at least one of the measurement or injection electrodes being suspended from said cable and placed in said borehole, said suspended electrode having an outer surface that is substantially cylindrical and electrically conductive and having a sufficient length and a diameter matching that of said borehole and capacitively coupled with ground surrounding said borehole; means for displacing said suspended electrode in the borehole by means of the cable; means for measuring a position of said suspended electrode in said borehole; means for measuring current flowing through said at least two injection electrodes; means for measuring a potential difference between the at least two measurement electrodes; and means for processing said current measurement and said potential difference measurement to deduce therefrom characteristics of subsoil in which the borehole is situated, for said measurement position, wherein said substantially cylindrical electrode is an injection electrode, and said injection electrode and a current measuring device are mounted on a common component for placement in the borehole. 11. Apparatus according to claim 10, comprising a second injection electrode for placement in the borehole, wherein said secon