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

Dynamo: is a device for converting mechanical energy into electrical energy. An electrical conductor moves across a magnetic field and electric current is induced. There is a powerful field magnet; between the poles of which the armature is rotated. This mechanical energy is converted into electrical energy In the form of a current in the armature.

Hydel project: waterfalls flowing on turbines, which are power units working on the principle of the water wheel, convert kinetic energy into mechanical energy. The turbines are connected with the armatures of dynamos and electricity is generated.

Transformer: It is a device by which an alternating current is changed to a different voltage without any change in the frequency of the current. Step-down transformer decreases the voltage.

Ink-filler: A partial vacuum is created inside the rubber tube by raising the level of the ink-filler which causes atmospheric pressure on the surface of the ink in the ink-pot to push the ink into the tube.