Introduction

Magnetic susceptibility corresponds to the ability of a material to be magnetized in an external field. Bulk (volume) susceptibility χ_vol can be directly related to the relative permeability (μ_r ) of a material: χ_vol = μ_r - 1 and where μ_r is the ratio of permeability of material/permeability of vacuum.

Figure 1. Magnetic susceptibility of common rocks (from Bartington User Manual)

Instrument settings

Instrument units 

The instrument is pre-set to display the susceptibility value directly in either SI or CGS units.

Numerical conversion from SI to CGS units (Table1) is accomplished by dividing the SI value by 4π, i.e. χ_CG = χ_SI/4π. The MS2 meter performs this function internally but by using the constant 0.4π to keep the numbers in a similar range of magnitude.

Table 1. Mass and volume magnetic susceptibility in SI and CGS units (from Bartington User Manual)

Instrument range selection

The instrument is pre-set to display the susceptibility value in either the 1.0 or 0.1 range. Table 2 shows the exponent value of the least significant digit (i.e. the last digit on the LCD) for each range. The units in Table 2 is for volume susceptibility.

Table 2. Exponent values for least significant digit in 1.0 and 0.1 ranges (from Bartington User Manual)

On the 0.1 range a decimal point will be displayed, effectively moving all the digits on the LCD to the left when compared with the 1.0 range. It should also be noted that one measurement in the 0.1 range will be an average of multiple readings taken over a period of 10 seconds.

MS2 Meter

The MS2 meter has front panel controls (Figure 2) and a rear panel (Figure 3).

Front panel (Figure 2):

1. ‘Measure’ push button, labelled ‘M’: permits sample readings to be taken.
2. Toggle switch: performs the same function as the push button but permits continuous measurements.
3. ‘Zero’ push button, labelled ‘Z’: permits ‘air’ readings to be taken. By performing a measurement to ‘air’ this control re-sets the instrument and brings subsequent measurements within the range of the display.
4. On/Off Switch: controls the internal battery supply and also permits the selection of either SI or CGS units.
5. Range multiplier switch: allows selection of either x1 or x0.1 sensitivity range. In the second case the result is shown to the first place of decimal and a 10-fold increase in measurement time provides additional noise filtering. The switch also activates the battery indicator.

Figure 2. MS2 meter, front panel.

Rear panel (Figure 3):

1. Power supply connector: the power adaptor plug should be inserted here.
2. RS-232 serial interface: an MS2 RS-232 cable can be inserted here and connected to a computer to allow control and communication using Bartsoft software.
3. Three-way rotary Switch: allows the user to select one of three RS-232 communication settings options.
4. Charging light: this LED will light up when power is connected and the MS2 battery is being charged.

Figure 3. MS2 meter, rear panel.

Taking measurements

Connecting the sensor

Connect the sensor to be used to the front panel socket of the MS2 meter with the supplied 50Ω TNC-TNC cable, and switch on by selecting either SI or CGS units (Figure 4).

Figure 4. Connecting a sensor to the MS2 meter

Measuring a sample

Switch on the instrument and allow ten minutes settling time before commencing measurements.

To measure a sample, follow the steps below (Figure 5):

1. Set the meter to the 1.0 range for the initial measurement of any sample, to establish the approximate result, before switching to the more sensitive 0.1 range. If the value of a sample is greater than 1000 then the most significant digit will not be seen if measured on the 0.1 range, leading to an apparent gross error in the result (Step #1).
2. Take an ‘air’ reading by pressing the ‘Z’ push button with the sample to be measured away from the influence of the sensor (Step #2). The display will appear blank and a colon will appear to show the instrument is ‘busy’. The completion of a Z cycle will be announced by a bleep and the display will show all zeros.
3. Place the sample within the influence of the sensor (Step #3) and press the ‘M’ button (Step #4). The ‘busy’ colon will appear and the display will show the previous reading until updated at the completion of the current measurement period. This is announced with a beep. At this time serial data will be transmitted if you use a computer interface. Note the value on a spreadsheet otherwise.
4. To measure weakly magnetic material, select the more sensitive x0.1 range and compensate for any thermally induced drift by making a series of measurements (recommendation: at least three measurements):
   a) Zero to air by pressing ‘Z’ button = R₀ ( = 0)
   b) Measure samples = R₁ , R₂ ....
   c) Measure ‘air’ = R_Final
   The mean of an ‘air’ measurement before and after the sample is subtracted from the nth sample measurement R_n:
   Corrected value R_k = R_n - n x (R_Final- R₀)/N
   where N is the total number of measurements after the initial zero (including R_Final)
   In the case that only one sample measurement is taken: R_k = R₁ - (R_Final- 0)/2
   When taking manual measurements, attempt to keep the time between measurements as consistent as possible. 
   The drift correction can be done automatically using Multisus or Bartsoft software where the time of each measurement is recorded, and the is drift applied linearly as a function of time.
5. Repeat Step #2 to Step #4 for every sample.

Figure 5. Sample measurement with the MS2C magnetic susceptibility meter

Calibration Check

A calibration check core is provided. The serial number of the MS2C sensor appears on the calibration core supplied. Our calibration check core is S/N 675, with K = 1451 x 10^-5 SI at 22°C (Figure 6A). The value for the core, when used with the specified diameter sensor (36 mm for the sensor on the JR), is printed around the middle of the core.
The stability of the sensor over time has been shown to exceed that of any core material which might be used routinely to check the calibration. The calibration check core should be used only to identify when some catastrophic calibration error has occurred. Last calibration check was on August 22, 2022.

Place the calibration check core in the sensor with the center line in the middle of the loop (Figure 6B). 

If the sensor is within its factory set calibration then the value obtained should be within 5% of the value printed on the core.

Figure 6. A) Calibration check core available on the JR and B) how to set the calibration core in the sensor loop

Correcting the magnetic susceptibility value

The value measured by the MS2C meter needs to be corrected to estimate true values of susceptibility (χ_vol) for narrow strata, i.e., for discrete samples.

Figure 7 is the calibration graph for the MS2C sensor for varying core diameter. In the case of a discrete cube sample, d is about 20 mm.

χ_{vol corrected} (SI) = (R_k) x 0.324 x 10^-5 

where R_k is the mean of measured values corrected to air (see Measuring a sample above)

Figure 7. Variation in the calibration value of χ_vol for varying core diameter d. d = core diameter (= 20 mm) and D = MS2C aperture (= 36 mm) + 8 mm. The measured value will be equal to χ_vol when d/D = 0.66. For other ratios of d/D the relative response will approximate to χ_rel = 3.45 * (d/D)³

Magnetic susceptibility measured by the MS2C meter can be compared to the bulk susceptibility measured by the MFK2: 

Bulk susceptibility (MFK2) = χ_{vol corrected} (SI) MS2C x sample volume (cc)

where sample volume is 10 cc.