Crompton 007 AC - Frequency Meter The Crompton Instruments AC synchroscope measures and displays the frequency difference of two power sources. Monitoring the display allows the user to connect two synchronized AC power systems together. This can help prevent the potential damage caused by connecting two unsynchronized power sources. The 4.5″ meter complies with ANSI C39.1 specifications. Terminal Cover is included with unit at no extra charge. Optional Neoprene Panel Gasket is also available readily from stock.