Mains joulemeter rvfm 614 007 the mains joulemeter wattmeter is an accurate light and compact unit it is designed for class demonstration and also for student use the meter measures the energy or power delivered to a mains appliance current transformers and sampling voltage transformer isolate the unit from the mains br br can be used to compare the efficiency of different types of lamp bulbs compare power consumption for lamp bulbs at different voltages variable mains transformer