Diesel and petrol engines: Diesel engine and petrol engines are both internal combustion engines. In a diesel engine vaporized diesel oil is mixed with the air which is ignited by the heat of light compression in the engine cylinder. The mixture, on the explosion, makes the piston move. The speed and power of the engine are controlled by the amount of fuel supplied. Diesel engines are used in heavy road vehicles and for industrial purposes because it is economical in operation. In a petrol engine, an explosive mixture of vaporized petrol and air is ignited by sparks in the

Cinema: is based on the principle of persistence of vision according to which the impression of an object upon the retina lasts for one-sixteenth of a second. The photographs of Successive positions of the moving object are taken on a transparent photographic film at a rate of 20 per second. The light is then cut off from the screen for 1/80th of a second and the film is drawn across the focal plane and replaced by the next portion. This remains stationary for the next 1 /20th of the second and the process continues. By this intermittent passing of

Radio communication: is the transmission of signals or messages by wireless by the radio transmitter. The radio transmitter emits a continuous career wave of definite constant amplitude and frequency. The sound impulses are converted into electrical impulses by means of a microphone. The career wave is modulated and these impulses superimposed upon it. The radio receiver is turned or adjusted to this modulated wave. The received Signal or wave is amplified and then rectified by transforming the alternating current induced in a conductor by the modulated wave into direct current by the rectifier. The current after further amplification is passed

Binocular: is a pair of tubes fitted with an objective and an eyepiece in which magnified images of distant and near objects are seen. Binoculars for distant objects are called opera glass. In prism binoculars, prisms are used for reducing the length of the scope tube. It consists of a pair of right-angled totally reflecting prisms. One prism reverses the image. formed by the objective, upside-down and the other from left to right.

The atomic-powered submarine uses atomic energy and can function for very long periods without refueling. Atomic energy is liberated by an atomic pile attached to the submarine. The liquid circulating through the atomic pile becomes very hot and the heat exchanger changes water into steam. This steam moves a turbine, connected to an electric generator and the electricity produced by the generator drives the submarine.

Radar: (acronym of Radio, Angle, Direction and Range) was designed to detect and find the range of moving objects by transmitting a beam of radio waves in the general direction of the objects and measuring the time taken for the reflected part of the wave (the echo) to return to the source of transmission. It gives the location, velocity, shape size, and nature of stationary and moving objects. Navigators on the sea, air and moving vehicles, missiles and artificial satellites all use radar

Telephone: is an apparatus used for the transmission of sound. It consists of a transmitter and a receiver connected by an electrical conductor. The transmitter is a carbon microphone, by means of which variable electrical impulses are caused to flow through the circuit. In the receiver these flow through a pair of coils of wire and these produce sounds.

Telegraph: is a system of transmitting the message over a distance by electrical impulses through wires. By pressing a key at the transmitting station, a circuit is closed and a current flows through the conducting wire to the receiver. The dots and dashes of the Morse code are obtained by changing the length of time for which the current flows. At the receiving station, the feeble electrical impulses are made to operate a relay. It closes a local circuit carrying a large current. This current causes the dots and dashes to be automatically recorded.

Loudspeaker: is an apparatus in which electrical energy is converted into sound energy. Its sound output is audible over a large area. Current is passed through a small voice coil fitted to the apex of a conical paper diaphragm. The voice coil travels in an angular gap over the middle arm of an E shaped permanent magnet. The voice coil is held in position in the center of the gap by a flexible plate. When an alternating current passes through the coil the paper cone diaphragm vibrates at the same frequency and emits sound waves. Since the diaphragm is

Tape recorder: is a system of recording and reproducing sound which uses a magnetic tape. When the recorder operates the tape runs through a recording head having electro-magnets. Sound waves pass through a microphone connected to the recording head. These sound waves cause the electric currents in the head to vary or fluctuate and thus set up a magnetic pattern. The pattern is impressed on an iron wire or metal-coated tape. When the tape is played back the magnetic pattern produces varying electric currents in the coils of the head. The varying electric currents are amplified and converted into sound