# Dictionary:chargeability (M)

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One of several units of induced polarization in the time domain.

1. The ratio of initial decay voltage (or secondary voltage) to primary voltage.

2 The dimensionless induced-polarization parameter of a material in which there is an induced-current dipole moment per unit volume P energized by a current density J:

${\displaystyle P=-MJ}$.

3. The fractional change in resistance measured on a decay curve, as a function of time:

${\displaystyle M={\frac {\delta \rho (t)J}{\rho }}}$.

4. The integrated area under an IP decay curve between times t1 and t2, normalized by the primary voltage Vp. Units are millivolt-seconds per volt.

${\displaystyle M={\frac {1}{V_{p}}}\int V_{t}dt}$.

For standardization, on-time and off-time may be indicated by subscripts, that is, 33M1 or M331 means "current on for three seconds and decay is measured for the first second of a three-seconds off time." Field measurements of chargeability are usually calibrated to the M331 standard, which differs by a factor of about 1000 from the value of M given in definitions 1 and 2 above.

5. The quantity described above multiplied by the conductivity, often expressed in millifarads/unit length. Called specific capacity when measured in farads per meter.

6. Chargeability can be related to frequency effect (q.v.).