Megohm Bridge method for measurement of resistances:
Fig a) shows the very high resistance with terminals A and B, and a guard terminal, which is put on the insulation. So it forms a three terminal resistance.
megohm bridge three terminal resistance
Let us consider take the hypothetical case of a 100 Mohm resistance .let us assume that this resistance is measured by an ordinary Wheatstone bridge. It is clear that Wheatstone will measure a resistance of 100*200/(100+200)=67Mohm instead of 100Mohm thus the error is 33 percent.
Megohm Bridge method for measurement of resistances
However if the same resistance is measured by a modified Wheatstone bridge as shown in fig b)with the guard connection G connected as indicated, the error in measurement will be redused and this modified Wheatstone bridge is called megohm bridge.
