Ask mr Brain...all will be explained

PUBLISHED : Tuesday, 20 July, 1999, 12:00am
UPDATED : Tuesday, 20 July, 1999, 12:00am

How do telephones work? The telephone is one of the simplest modern devices. A very basic telephone would have three parts, a switch that connects and disconnects the phone from the network - generally called the hook switch, it connects when you lift the handset; an 8-ohm speaker, and a microphone which, in the past was as simple as carbon granules compressed between two thin metal plates. Sound waves from your voice would compress and decompress the granules changing their resistance and modulating the current flowing through the microphone.

In a modern phone, there is an electronic microphone, amplifier and a circuit in place of carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to give a pleasant ringing tone.

When manual switchboards were used a long time ago, a pair of copper wires running from a house would connect it to a central office in the middle of town.

A switchboard operator sat in front of the board with one jack for every set of wire.

Above the jacks, small lights lit up when someone picked up the handset on their telephone. The hook switch completed the circuit and lit up the small lights.

The operator picked up the handset and asked callers who they wanted to speak to. The operator would then send a ring signal to recipients and wait for them to pick up the phone.

In systems we use today, the operator has been replaced by an electronic switch and when you pick up the phone, the switch senses the completion of your circuit and gives out a dial tone to let you know your phone is working properly so that you can go ahead and dial.

Where are the highest and lowest temperatures found in the world? Why do temperatures vary in different places? YINDY CHOW SIU-YIN Yuen Long Merchants Association Secondary School The lowest temperature ever recorded was in Antarctica, -89.2 degrees Celsius. It was recorded in 1983, at the Russian Base, Vostok. In addition to being the coldest place on earth it is also the wettest and the driest! Ninety per cent of Antarctica is covered by ice, and Antarctica contains 70 per cent of the Earth's fresh water and 90 per cent of its ice. It is also the driest place on Earth because Antarctica is technically a desert and receives no less than five centimetres of precipitation a year - about the same as the Sahara Desert.

The highest temperature recorded 57.7 degrees Celsius was in Africa, Libya in Al'Aziziyah, on September 13, 1992.

The lowest temperature technically possible is -273.15 degrees Celsius, known as absolute zero.

Temperatures of different regions vary according to their proximity to the sea and the equator, altitude and many other factors.

For example, the Arctic circle is cold because it is far away from the equator which receives the most of the sun's direct energy, but the Himalayas are also very cold because of its high altitude.