High impedance inputs are preferred on measuring instruments such as voltmeters or oscilloscopes. They are available for both voltmeters and oscilloscopes. Solenoid voltmeters are useless on low-voltage circuits (for example, 12 volt circuits). Solenoid voltmeters draw appreciable current in operation. It contained Weston ammeters and voltmeters and Westinghouse circuit breakers. Electronics hobbyists were moving on to digital projects such as digital voltmeters and frequency counters. The Galvani potential difference is not directly measurable using voltmeters. Later the company developed miniature digital voltmeters and frequency counters. A similar distinction can be made between electroscopes, electrometers, and voltmeters for voltage measurements. Measuring instruments such as ammeters and voltmeters need to be checked periodically against known standards. Their use is typically limited to digital voltmeters and other instruments requiring highly accurate measurements. Many digital voltmeters and all moving coil meters are in this category. Voltmeters are made in a wide range of styles.