724. 



ELECTRICAL METHODS 



[Chap. 10 



companies to test the grounds of power stations and transmission towers, 

 and the like. The Megger differs from the Gish-Rooney arrangement in 

 two respects: (1) power is supphed from a generator mounted with the 

 commutator on the same shaft-; (2) measurements of voltage and current 

 are made with cross-coil instruments so that automatically the ratio of 



Hi 



-^^^; — ;*-- 



60 120 6,^^4 



Fia. 10-59o. Circuit of Leeds and Northrup potentiometer. 



Colorado School of Mines 



Fig. 10-596. Close-up of double commutator. 



voltage and current and therefore the resistance is determined. In the 

 scheme of Fig. 10-60 the cross formed by the current and potential coil is 

 not shown. The current coil is in series with the current or external leads, 

 while the potential coil is across the internal or potential pair of electrodes. 

 Although the potential coil is in series with a high resistance, this is gen- 

 erally not sufficient to give the same accuracy as the potentiometer method. 

 Therefore, resistivities determined with the Megger are generally lower 



