Calibration and resolution-determining apparatus for dial indicators is disclosed. A control unit calculates a required output of an inverter from an input accelerator travel. The battery charger has a primary coil enclosed within a housing, and which is coupled to a power source. The solid particulate material may be selected from materials having of specific gravity of at least 2. The two input driving sections provide for the application of phase-modulated input signals to control the gain or amplitude of the transformer output.