My Science School

          An automobile's speed is measured by a speedometer fitted next to the steering wheel of a car. It indicates the vehicle’s speed in kilometers per hour or miles per hour. The speed is read on the dial which is numbered from 0 to 160, by means of a pointer. Most speedometers also incorporate an odometer – a device that records the distance travelled by the vehicle.

          A speedometer is driven by a flexible cable that is connected to a set of gears in the vehicle’s transmission. When the vehicle moves, the gears turn a core or flexible metal shaft inside the cable. The core turns a magnet inside a metal drum called a speed cup. This is located inside the speedometer housing. The revolving magnet exerts a turning force on the speed cup. In turn the speed cup is held back from revolving freely with the magnet, by the opposing action of a hairspring. The movement of the speed cup is transferred to the pointer on the dial. The hairspring brings the pointer back to ‘zero’ when the vehicle stops moving. Most of the speedometers register 36 km/hr when the core inside the cable revolves at 1000 revolutions per minute.

          The odometer registers total kilometers travelled by the vehicle. Some automobiles also have Trip odometers that can be reset to ‘zero’ at the beginning of a particular trip. An odometer consists of a chain of gears (with a gear ratio of 1000 : 1) that causes a drum, graduated in 10th of a mile or kilometer, to make one turn per mile or kilometer. A series, commonly of six such drums, is arranged in such a way that one of the numerals on each drum is visible in a rectangular window. The drums are coupled so that 10 revolutions of the first cause one revolution of the second and so forth, the numbers appearing in the window represent the accumulated mileage.

Electricity is supplied to our homes, schools, factories and stores through copper or aluminium wires from power stations. These power stations burn coal or oil, use nuclear reactions or the energy of falling water to produce energy to run the generators. The power thus generated is then transmitted to different cities and places where it is required. Electricity is then transmitted through transmission lines.

To avoid the loss of power, the output voltage from the generator is first stepped up to a high voltage by a step-up transformer. After being received at the city power station, it is again reduced to low voltage, before it reaches our homes or factories. Now question arises how is electricity conducted through wires?

We know that all substances are made up of atoms. Materials which allow the passage of electricity are called conductors. Metals, such as copper, aluminium, silver and gold are good conductors of electricity. The atoms of these metals have loosely bonded electrons. These electrons are free to move within the metal. These are called free electrons and are responsible for the conduction of current. More the number of free electrons in the metal, better it conducts the electricity. 

When electric battery is connected across the ends of the metal wire, the negatively charged free electrons move away from the end connected to the negative terminals and flow toward the positive terminal. This flow of electrons is nothing but the electric current. Hence the drifting electrons cause electricity to flow. Greater the number of free electrons in a metal, easier it is for electricity to flow through it.

Some materials are poor conductors of electricity because they have less number of free electrons. Poor conductors resist the flow of electricity. The resistance of a wire depends upon its material length and area of its cross-section.

Some substances do not allow electricity to flow through them and are called insulators. These substances contain tightly-bonded electrons that cannot move away from atoms. Hence they do not conduct electricity. Glass, mica, wood, plastic and rubber are common insulators. Some substances like silicon and germanium are neither good conductors nor insulators. They are called semi-conductors.