HI-3604 is a low-frequency field strength meter used to measure the strength of an electromagnetic field, used for 50 or 60 Hz power system transmission lines and related electrical equipment or instrument electromagnetic field strength measurement and evaluation. The device can directly read the field strength by digital display, or read the data indirectly through the optical fiber remote control device (HI-3616). HI-3604 can be applied to scientific research or environmental research fields, such as understanding the field strength under the grid frequency of a certain area. HI-3604 provides a powerful tool for engineers, industrial hygienists and personnel to observe environmental electromagnetic parameters at the frequency of the power system.