Strain Guage

Strain is defined as the amount of deformation in a body due the applied force.  The Strain gauge is a thin layer of metal in the form of a foil and mounted on a flexible strip.  The strip is mounted on the specimen whose strain is to be measured.  The mounting of the strain gauge of the specimen is done through a special adhesive to ensure that the strain of the specimen is accurately transferred to the strain gauge.

When the specimen is subjected to a force, it elongates.  This elongation is also transferred to the strain gauge which is mounted on the specimen.  This changes the resistance of the gauge which is measured as the amount of deformation.

Since the value of the change is resistance is very small, conventional methods of measuring the resistances cannot be used.

Hence, a Wheatstone bridge excited by an external power supply is used, the resistance of the strain gauge is one arm of the Wheatstone Bridge.

Heating Elements - An Overview

A heating element is a device which converts electricity into heat.  Heating elements work on the principle of Joule heating which is also known as Resistive heating.  When electric current flows through a conductor, the electrons collide with atoms in the lattice structure producing heat.  Electrical Energy is thus converted into heat energy.  

The heating elements can get oxidized at high temperatures.  Hence, the heating element needs to be made of a material which can withstand the high temperatures.

Heating elements are widely used in both domestic and industrial applications.  Toasters, kitchen ovens, heating irons, water heaters and industrial furnaces are some equipments where heating elements are used.

The element is constructed by tightly coiled conductors of special alloys such as Nichrome.  Heating elements are simple in construction and operation.  Heating elements are usually controlled by a thermostat which measures the temperature and switches off the heating elements when the set temperature has been reached.

Heating elements are classified as convective elements and Radiative Elements.  Convective elements work by transferring heat through air to the medium to be heated.  Radiative Elements function by transferring heat through infrared radiation.

Curie Temperature

Curie Temperature is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic.  A ferromagnetic material is one in which the individual magnetic moments of the atoms get aligned in the presence of an external magnetic field.  

When the temperature is raised above the curie temperature, the thermal agitation of the atoms result in the random alignment of the individual magnetic moments.  This results in the material becoming paramagnetic.  A ferromagnetic material which is above its curie temperature will not be attracted by a magnet.

Check out this video to see a ferromagnetic material which is above the curie temperature get attracted by a magnet as soon as it cools below the curie temperature.

The Curie Temperature is also known as the Curie Point.

The Curie Temperature is used in storage devices such as magneto-optical storage systems to erase and form new memory.

Space Heaters

Space heaters are used to heat small confined spaces.  Space heaters are used widely in industries in panels and in electric machines such as alternators and motors to prevent condensate formation on the winding.

The space heaters should be switched as soon as the alternator or the motor comes to a stop.

Space heaters enable localised heating of an area to a specific degree. Space heaters for industrial application are electric heaters.  Space heaters work on the principle of convection or radiation.

Convective heaters work by first heating the air which transfers the heat to the surrounding area.  Radiative heaters work by emitting infrared radiation which heats the surface on which it falls.  Infrared heaters work faster and can be used to heat a specific location.  

Nichrome - An Overview

Nichrome is an alloy of Nickel and Chromium.  Nicrome is used widely in heating elements.  It has a high melting point of 1400 degrees celsius.  

Nichrome is resistant to oxidation and is stable at high temperatures.  When heated, Nichrome forms a layer of Chromium oxide on the surface which prevents further oxidation.  

Following are some of the physical properties of Nichrome.

Electrical Resistivity at room temperature                     : 1.0 x 10-6 to 1.5 x 10-6 ohm m

Thermal Conductivity                         : 11.3 W/moC

Magnetic Attraction                                 : None

Thermal Expansion Coefficient (20oC to 100oC)         : 13.4 x 10-6/oC

Temperature Coefficient of Resistivity (25oC to 100oC)   : 100 ppm/oC

Specific Gravity                                 : 8.4

Density                                 : 8400 kg/m3

Melting point                                   : 1400oC

Specific Heat                                 : 450 J/kgoC

Modulus of elasticity                                 : 2.2 x 1011

Temperature Coefficient of Resistance

Temperature Coefficient of Resistance refers to the variation in resistance in response to increase in temperature (per unit kelvin). 

When the temperature of a material is increased, the atoms are agitated due to the increase in heat energy.  This agitation causes an increase in resistance to the flow of electrons.  This causes the resistance of a material to increase.  This is termed as positive coefficient of Resistance.  

The resistance of a material at a given temperature can be determined by the following formula

Where Ro is the resistance of the material at zero degree temperature.  α  is the temperature coefficient of resistance while T is the temperature in kelvin)

Negative Temperature Coefficient of Resistance

Some materials also display a negative temperature coefficient of resistance  i.e. the resistance decreases with increase in temperature.  Carbon, Silicon and Germanium are elements with negative temperature coefficient of resistance. Insulators such as plastics, rubber, etc also exhibit negative temperature coefficient of resistance.  

Constantan - An Overview

Constantan is an alloy consisting of copper (55%) and Nickel (45%).  It has a very low temperature coefficient of resistance which makes it ideal for use in instruments and bridges.  Constantan also has good mechanical properties such as good fatigue and elongation properties.

Constantan has better corrosion resistance than manganin.

Constantan is also used as the negative element in J and T type thermocouples.  Constantan has high resistivity.  This makes it ideal for use in resistance grids.  

Constantan is also used in the construction of industrial rheostats and wire wound precision resistors

PHYSICAL PROPERTIES

Density                                                              - 0.321 lbs/cu.in.

Melting Point (Approx.)                                     - 1210° C

Electrical Resistivity @ R.T.                               - 50.8 Microhm· cm

Temperature Coefficient of Resistivity (TCR)       - ± 30 PPM/° c
(25°C to 105°C)

Thermal Expansion Coefficient (20°C to 100°C)  - 14.9 x 10-6/°C

Thermal EMF vs. Copper (0°C to 100°C)           - -0.043 Millivolts/°C

Thermal Conductivity R.T.                                   - 21.2W/m· K

Magnetic Attraction                                             - None

Specific Heat                                                       - .094 gram· cal./°C

BH Curve - A brief description

The BH Curve is a plot of the Magnetic Flux Density (B) versus the Magnetic Field Strength (H).  This is an important curve in selecting materials for electric machines.  The curve tells us about the change in the Flux density of a material as the magnetic field strength is increased.

When the magnetic field strength is increased gradually, the domains inside the material exposed to the field get aligned gradually.  This results in the increasing flux density of the material. As the magnetic field strength is increased further, there comes a point where the curve flattens.  This implies that the magnetization is complete and further increase in the flux density is not possible.  This is the point of maximum positive saturation, b in the curve.

If the magnetic field strength is decreased at this time, shown by section b–c , it is observed that the magnetic flux density does not come to zero even after the magnetic field strength has been reduced to zero. 

This indicates that the material is able to retain the magnetism.  This is known as residual magnetism.

In order to bring the residual magnetism to zero, the magnetic field strength is applied in the opposite direction such that H is now negative.  

This is shown in the region of the curve c–d.  The magnetic field strength required to bring the flux density to zero is known as the coercive strength of the magnetic field.  

If the magnetic field strength is increased in the opposite direction, the magnetic flux density also increases until it reaches negative saturation, point e.  If the magnetic field strength is brought to zero, the material still retains magnetism in the opposite direction.  This is known as the negative residual magnetic strength, indicated by point f.  

If the magnetic strength is increased in the positive direction, the magnetic flux density becomes zero once again.  This is point g of the curve.  

The BH curve is also known as the hysteresis curve as the reversed curve does not follow the original curve.  

BH curves help compare the magnetic properties on one material over another while selecting a material for a specific application.  

Manganin Alloy - An Overview

Manganin is an alloy of copper (86%), Manganese (12%) and nickel (2%).  Manganin is widely used in the construction of resistors as it has near zero temperature coefficient of resistance which means that its resistance does not change for an increase in temperature. 

The temperature coefficient of Resistance of Manganin is 

0.00001

This is an vital aspect when designing resistors used in measurement and control circuits. 

Until recently, Manganin was used in designing laboratory standards for the ohm. 

Manganin was developed by Edward Weston in 1892.