Introduction to Basic Manufacturing Processes and ... - always yours

118 Introduction to Basic Manufacturing Processes and Workshop Technology
4. Resistivity
It is the property of a material by which it resists the flow of electricity through it.
5. Thermoelectricity
If two dissimilar metals are joined and then this junction is heated, a small voltage (in
the milli-volt range) is produced, and this is known as thermoelectric effect. It is the base of
the thermocouple. Thermo -couples are prepared using the properties of metals.
7.1.5 Magnetic Properties
Magnetic properties of materials arise from the spin of the electrons and the orbital motion
of electrons around the atomic nuclei. In certain atoms, the opposite spins neutralize one
another, but when there is an excess of electrons spinning in one direction, magnetic field
is produced. Many materials except ferromagnetic material which can form permanent magnet,
exhibit magnetic affects only when subjected to an external electro-magnetic field. Magnetic
properties of materials specify many aspects of the structure and behavior of the matter.
Various magnetic properties of the materials are magnetic hysteresis, coercive force and
absolute permeability which are defined as under.
1. Magnetic Hysteresis
Hysteresis is defined as the lagging of magnetization or induction flux density behind the
magnetizing force or it is that quality of a magnetic substance due to energy is dissipated in
it on reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the rising
temperature at which the given material ceases to be ferromagnetic, or the falling temperature
at which it becomes magnetic. Almost all magnetic materials exhibit the phenomenon called
hysteresis.
2. Coercive Force
It is defined as the magnetizing force which is essential to neutralize completely the
magnetism in an electromagnet after the value of magnetizing force becomes zero.
3. Absolute Permeability
It is defined as the ratio of the flux density in a material to the magnetizing force
producing that flux density. Paramagnetic materials possess permeability greater than one
whereas di-magnetic materials have permeability less than one.
7.1.6 Optical Properties
The main optical properties of engineering materials are refractive index, absorptivity,
absorption co-efficient, reflectivity and transmissivity. Refractive index is an important optical
property of metal which is defined as under.
Refractive Index
It is defined as the ratio of velocity of light in vacuum to the velocity of a material. It
can also be termed as the ratio of sine of angle of incidence to the sine of refraction.
7.1.7 Mechanical Properties
Under the action of various kinds of forces, the behavior of the material is studied that
measures the strength and lasting characteristic of a material in service. The mechanical
properties of materials are of great industrial importance in the design of tools, machines and