542 



EXPLORATION GEOPHYSICS 



The Megger 



The Megger is a trade name applied to a type of instrument ordinarily employed 

 for testing electrical insulation resistance and ground resistance. A modification of 

 this instrument, the "Megger Ground Tester," may be used for shallow subsurface 

 investigations. 



GENERATOR 



Fig. 331. — Diagram of connections for Megger circuit. 



The Megger Ground Tester f is illustrated diagrammatically in Figure 331. 

 Turning the crank rotates a small direct-current generator. The output from this 

 generator passes first through the current coil or ammeter element of an ohmmeter ; 

 the current then goes to a commutator mounted on the same shaft as the generator 

 and is changed into commutated current of about 50 cycles per second. The current 

 binding posts Ci and C2 are connected to the two energizing stakes. The two potential 

 stakes are connected to the potential binding posts Fi and Fa. The potential drop 

 across these two stakes is measured by passing the current picked up by them through 

 a second commutator, run synchronously with the first, which converts the current 

 back into a uni-directional flow. The current then goes to the potential coil, or volt- 

 meter element, of the ohmmeter. 



The current coil and the potential coil of the ohmmeter are mounted on a common 

 spindle (not shown in the sketch). The mounting is such that the torques exerted 

 by the current and potential coil on the moving element of the ohmmeter oppose 

 one another. Hence, the deflection of the ohmmeter element is proportional to the 

 quotient of the potential divided by the current, and the scale of the Megger may be 

 calibrated to read volts divided by amperes, i.e., ohms, directly. The instrument 

 therefore indicates the effective resistance between terminals Pi and P2. 



Before calculating apparent resistivity values, various corrections must be applied 

 to the E/I quotient. A small but appreciable current flows in the so-called potential 



t B. Low, S. F. Kelly, W. B. Creagmile, "Applying the Megger Ground Tester in Electrical 

 Exploration," A.I.M.E. Geophysical Prospecting, 1933, pp. 114-125. 



