Mega Ohm Bridge

Methods for Measurement of High Resistance:

The different methods employed are :
1. Direct deflection method.
2. Loss of charge method:
3. Megohm bridge.
4. Meggar

Megohm bridge

Figure (a) shows very high resistance R with its two main terminals A and B, and a guard terminal, which is put on the insulation. This high resistance may be diagrammatically represented as in Figure(b). The resistance R is between main terminals A and B and the leakage resistances RAr. and Rnr between the A and B of the main terminal from a "Three terminal resistance".

Let us consider the hypothetical case of a 100 M Ohm resistance. We assume that each of the leakage
resistances is 100 M Ohm i.e., RAG = RBG = 100 M Ohm. Let this resistance be measured by an ordinary
Wheatstone bridge as shown in Figure (a) below It is clear that the Wheatstone bridge will measure the resistance of (100 x 200) / (100 +200)= 67M Ohm instead of 100 M Ohm thus giving an error of 33 per cent.

However, if the same resistance is measured by a modified Wheatstone bridge as shown in Figure (b) above with the guard connection G connected as indicated, the error in measurement is considerably reduced. For the arrangement shown in Figure(b) resistance RBG is put in parallel with the galvanometer and thus it has no effect on the balance and only effects the sensitivity of the galvanometer slightly. The resistance Rag =100M Ohm is put in parallel with a resistance P = 100 k Ohm and therefore for the arrangement shown the measured value has an error of only 0.01 percent and this error is entirely negligible for measurements of this type.

The arrangement of the Figure illustrates the operation of a Megohm bridge. The above Figure shows the circuit of a completely self-contained Megohm bridge which includes power supplies, bridge members, amplifiers, and indicating, instruments. It has a range from 0.1 M Ohm to 106 M Ohm. The accuracy is within 3% for the lower part of the range to possibly 10% above 10,000 M Ohm.

Sensitivity for balancing against high resistance is obtained by the use of adjustable high voltage supplies of 500 V or 1000 V and the use of a sensitive null indicating arrangements such as a high gain amplifier with an electronic voltmeter or a C.R.O. The dial on Ohm is calibrated 1 - 10 - 100 - 1000 M Ohm, with main decade 1 - 10 occupying the greater part of the dial space. Since unknown resistance R = PS/ Q the arm Q is made, tapered, so that the dial calibration is approximately logarithmic in the main decade, 1 — 10. Arm S gives five multipliers, 0.1, 1, 10, 100 and 1000. The junction of ratio arms P and Q is brought on the main panel and is designated as the 'Guard' terminal.

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