Stroboscopic Effect in Fluorescent Lamps

The Stroboscopic Effect in Fluorescent lamp is a phenomenon which causes running or moving equipment to appear stationary or appear to be operating slower than they actually are.

In an AC supply, the voltage drops 100 times a second to zero volts as the supply frequency is 50 Hz.  When a Fluorescent lamp is operating with an AC supply, the light intensity drops 100 times a second.  This flicker is not noticeable to the human eye due to the persistence of vision. 

When a worker in a factory observes a running machine, say a flywheel under the illumination of a fluorescent light, the flywheel may appear to be stationary or to be operating at reduced speed.  This can result in accidents and is highly dangerous.

A sewing machine whose needle moves up and down may appear to be stationary and the operator can prick the fingers.  These are some examples where the stroboscopic effect in the Fluorescent lamps can prove to be dangerous.  When using fluorescent lamps around rotating or moving machinery, two lamps powered by two different phases should be used.  This ensures that both the lamps do not flicker due to the zero crossing at the same time. 

If another phase is not available, a capacitor can be added in series to one lamp.  This ensures that there is a phase lag between the two lamps. The Stroboscopic effect can be eliminated by  using electronic ballasts where the supply to the lamps is of a very high frequency of the order of kiloHertz.

Structure of an Optical Cable

Optical Cables are used extensively in the field of telecommunication.  They have numerous advantages over conventional communication on wires.  They are efficient, quick and secure.  Optical Communication Cables are designed to provide high efficiency of transmission.  They are also designed to withstand the challenges of the external environment such as corrosion, heat and physical stress.
The Optical Cable has the following main components. 
Core
The Core provides the pathway for the light to travel.  It is made of glass or transparent plastic material. 
Cladding
The cladding is the layer that covers the core.  The function of the Cladding is to reflect the light which may come out of the core back into the core.  This results in Total Internal Reflection which ensures that there is no loss of the light signal. 
Buffer
The Buffer is a coating which is outside the Cladding.  The Buffer serves to protect the optical fibre. 
Aramid Yarn Protection
The Aramid yarn which surrounds the buffer provides crush protection to the cable.
Protective Jacket
The protective jacket offers mechanical protection to the cable. 

How do Electronic Ballasts in Fluorescent lights work ?

A Ballast in a fluorescent light is necessary to get the light glowing.  The Ballasts generates a high voltage by means of the would coil which acts as an inductance.  When the starter interrupts the supply to the inductance, a powerful voltage is generated.  These are called magnetic ballasts. 

However, magnetic ballasts take time to get the lamps to start.  They also have an undesirable hum.  They also produce a flicker before the tube lights up continually.

Electronic eliminate the problem of initial flicker and hum.  They also reduce power consumption.
Electronic Ballasts work by converting the AC supply into DC first.  This rectified DC is then chopped by a chopper circuit to generate high voltages to start the discharge in the lamp.  The chopped AC waveform is t a very high frequency of the order of kHz.  This ensures that the flicker is reduced to a minimum.

The efficiency of the lamp is also increased.  Electronic Ballasts are 10% more efficient than magnetic ballasts. Electronic Ballasts can generate harmonics.  This, however, is insignificant as the amount is very small.

Why does the resistance of metals increase when they are heated ?

Conductivity in metals is due to the presence of free electrons in the atomic lattice.  When the metal is heated, the atoms in the lattice vibrate.  This results in reduced movement of the electrons as they hit against the vibrating atoms.

This results in an increase in resistance of the metals. 

Most Metals have positive temperature coefficient of resistance. There are , however, exceptions such as carbon and semiconductor metals such as Silicon and Germanium.  Some Alloys have zero temperature coefficient of temperature which means that the resistance does not change with increase of temperature.  Manganin is an example.

What is the effect of temperature on semiconductors ?

When a semiconductor is heated, the conductance increases and the resistance decreases.  Semiconductors, thus, have a negative temperature coefficient of resistance.
When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy.  These electrons are able to overcome the attraction of the nucleus and leave the atom.  Thus they become free electrons which can conduct. 
The number of electrons increase exponentially and this results in a large drop in the resistance. 
Electronic devices will behave erratically above a certain temperatures.  Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function. 
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band.