Ammeters, Meggers, and Wheatstone Bridge (A8003)

Learn about Wheatstone bridges, megohmmeters, and clamp-on ammeters.


This lesson is designed for participants familiar with AC and DC theory, electrical safety, and electrical print reading. A basic understanding of electronic devices and circuits is recommended.


This lesson describes Wheatstone bridges, megohmmeters, and clamp-on ammeters. This lesson provides examples of the use of these instruments, identifies their components, and defines their functions. This lesson also describes safety and selection considerations for their use, describes how to set up the instruments, how to connect them to the systems under test, and how to take and read measurements. This lesson describes how to take a resistance reading of a Three-phase AC motor with a megohmmeter, how to set mechanical and electrical zero on a Wheatstone bridge, and how to interpret a Wheatstone bridge reading. This lesson also defines the “record” and “lock” features of a clamp-on ammeter and describes how to modify the range of the meter for the best results.


Explain the use of a megger, identify basic components, define its function. Describe the safety and selection considerations for function, safety and selection for a clamp-on ammeter. Describe the procedures for setting up a clamp-on ammeter, know how to take a reading and modify the range.