13 Jul 2026

Magnetic Susceptibility of Solid Samples

practical ug-vi magnetism susceptibility solids

Experimental arrangement

The solid sample is positioned in the non-uniform magnetic field and the force or balance reading is recorded.

Aim

To determine and compare the magnetic susceptibility of solid samples by a force or Gouy-balance method.

Apparatus

Gouy balance, electromagnet, sample tube, analytical balance, power supply, and Gauss meter.

Theory

The force on a sample in a non-uniform field is related to its susceptibility. In the calibrated form of the apparatus,

\[\chi=C\frac{\Delta m}{H^2},\]

where $C$ is the apparatus constant, $\Delta m$ is the change in apparent mass, and $H$ is the field.

Observations

Sample	$H$ (A m$^{-1}$)	Apparent mass change (mg)	Susceptibility
Alum	$2.10\times10^5$	7.2	$1.7\times10^{-5}$
Ferric salt	$2.10\times10^5$	128.0	$3.0\times10^{-4}$
Bismuth	$2.10\times10^5$	-4.1	$-9.7\times10^{-6}$

Result

The ferric salt is strongly paramagnetic, alum is weakly paramagnetic, and bismuth is diamagnetic.

Precautions

1. Remove magnetic materials from the balance platform.
2. Keep the sample position fixed.
3. Record the field after it becomes steady.
4. Use an empty-tube correction.

Viva Questions

1. What is a diamagnetic substance? It is weakly repelled by a magnetic field and has negative susceptibility.
2. Why is a calibration constant used? The measured balance change is converted into susceptibility through calibration.
3. Why should the sample be dry? Moisture changes the mass and may contribute its own magnetic response.