Experiment No……..
ObjectiveTo study the Magnetostriction in metallic rod with Michelson Interferometer.
Apparatus RequiredHelium Neon Laser with adjustable stand, Constant Current Source, Solenoid, Screen, Michelson
Interferometer setup.
Screen
Mirror
M1
Screw to
adjusting
mirror
Lens
L1
LASER
Beam Splitter
Mirror
M2
Sample Rod
Solenoid
Terminal for
Connection
Fig.1. Michelson Arrangement
TheoryWith the aid of two mirrors in a Michelson arrangement, light is brought to interference. Due to
the magnetostrictive effect, one of the mirrors is shifted by variation in the magnetic field applied
to a sample, and the change in the interference pattern is observed.
On magnetization a magnetic material undergoes changes in volume which is small i.e. of the
order of 10-6. The ferromagnetic substances exhibit a lengthening and shortening parallel and
perpendicular to the direction of magnetization. Such changes are termed as positive and
negative magnetostriction. Due to magnetostriction a ferromagnetic substance can reduce its total
an-isotropic and elastic energy.
Magnetostriction. Magnetostriction Is the property of certain metals and alloys to change
dimension under the influence of a magnetic field.
The sample rod is attached with mirror M2 of Michelson interferometer. Therefore, the change in
𝑛𝜆
length l is given by l =
2
Where; n is the mean nos. of rings (i.e. nos. of fringes appeared and disappeared).
 is wavelength of laser light used.
Procedure1. Align the laser in such a way that the laser beam falls in the middle of mirror M1 in
absence of lens L1 and beam splitter.
2. Now insert lens L1 and try to bring laser beam again in the middle of mirror M1 with the
help of adjustable screw attached to lens L1.
3. Now insert the beam splitter at an angle of 450 and then rotate the beam splitter in such a
way that it reflects half of the incident light (50%) to mirror M1 and rest half (50%) to
mirror M2.
4. In doing so after reflection from mirror M1 and M2, you will get two luminous nearby
spot on the screen.
5. Try to coincide these spots by slightly adjusting mirror M1 with the help of the screws
available at the back side of this mirror.
6. Get the clear interference pattern on the screen by adjusting mirror M1.
Remark- These six steps for adjustment of Michelson interferometer, which is
already adjusted for interference pattern.
7. At first, switch on the laser only and see on screen you will get circular rings
(Interference pattern). If not, adjust the laser by the screw which is provided inside the
laser stand.
8. Now, connect the magnetostriction coil with the constant current source and apply some
current say 150 mA.
9. You will find the appearance of rings on the screen.
10. Count and note down the total nos. of rings appeared. This is the value of n1.
11. Now, switch off the current source.
12. You will find the disappearance of rings on the screen.
13. Count and note down the total nos. of rings disappeared. This is the value of n2.
14. Now you can calculate the value of n from the table and hence value of l from the
formula given at this current say 150 mA.
15. Similarly you can repeat the experiment for different values of coil current say at 250 mA
in order to calculate their change in length.
Note: - For each observation you have to count appearing or disappearing number
at least 20 minutes.
Observations:S.No.
Current
I (mA)
1.
150
2.
250
Nos. of Rings
Appearing
Disappearing
(n1)
(n2)
Mean
n=
(𝒏𝟏+𝒏𝟐)
𝟐
CalculationsThe change in length l is given by l = n/2
Where; n is the mean nos. of rings (i.e. nos. of fringes appeared and disappeared).
 is wavelength of laser light used.
(i) For magnetostriction coil current I = 150 mA
The mean of number of rings n = (n1+n2)/2 = ………….
Wavelength of laser light 𝜆 = 6328 Ǻ.
Change in length Δl can be calculated using above formula
Δl = n𝜆/2 = ……………..
(ii) Similarly we can calculate Δl for other value of coil current.
ResultThe change in length for different coil currents due to positive magnetostriction effect in
Iron rod is shown in the following tables:
Sr.No.
Magnetostriction
Coil Current I mA.
(i)
150
(ii)
250
Change in
length l mm
Precautions:1. Don’t disturb the setup as it causes hindrance in counting of the rings.
2. Lens and mirror should be properly cleaned.