How to use magnetism and electricity on machines. How an electric bell, telegraphy, telephone and electric motor work.
MAGNETISM AND ELECTRICITY AT WORK
Every time we use a car we need magnetism and electricity to get us going and keep us moving. When we listen to music on a radio electromagnetism has been used in various ways “before the sound reaches our ears. Many machines such as washing machines, hairdryers, mixers and power tools would not be much use without their electric motors. Generally, life is made more interesting and enjoyable due to the ways in which magnetism and electricity have been put to work for us.
Electrical machines which use the magnetic effect of an electric current usually have a coil which acts as an electromagnet.
THE ELECTRIC BELL
Fig shows the main parts of an electric bell. When you press the button B, current flows through the circuit and the soft iron core M becomes an electromagnet. The electromagnet attracts the armature which is also made of soft iron. The bell hammer is connected to a piece of springy steel attached to the armature. It strikes the gong. What happens to the circuit when the electromagnet pulls A towards it? The circuit is broken. Then what happens to the electromagnet? It loses its magnetism and releases the armature. The circuit is closed again and the cycle repeats itself rapidly as long as the push- button is being pressed.
One of the most useful and practical devices for communication is the telegraph. The first telegraph and code were invented by Samuel Morse in 1837. The code, called the Morse Code, was named after him.
In Fig, when the contact is closed in city A the current flows through the wires to a sounder in city B. There it passes through the electromagnet and back again to city A, through the earth. The armature is held down by the electromagnet as long as the contact is closed. As soon as the circuit is broken, the armature is pulled back by a spring. This movement of the armature in the definite pattern of the code is recorded on a piece of paper.
Alexander Graham Bell invented the first telephone in 1876. A telephone consists of three parts, microphone, an electric circuit and earpiece.
The microphone has a thin diaphragm that vibrates as sound waves hit it. Behind the diaphragm is a small box filled with carbon granules. The vibrations press on the carbon granules and this changes the resistance to the strength of the current. The vibrations are transmitted to the electric wire as a current. The current passes through the primary coil of a step- up transformer to a secondary coil where an alternating current represents the sound waves produced. This alternating current passes along the wires to the earpiece receiver.
The earpiece consists of a small permanent magnet which attracts a thin flexible steel diaphragm. There is a coil of wire around each pole of the magnet and it is through these coils that the current flows. The changing alternating current changes the strength of the magnetic field and causes the diaphragm to vibrate and give off sound waves that correspond to the original sound waves that hit the microphone.
An electric motor transforms electrical energy into mechanical energy. When a current flows through a coil in a magnetic field, the coil moves. The main parts of an electric motor are the armature, the magnetic field and the brushes. The armature is surrounded by a coil of wire and when electricity passes through it, it becomes an electormagnet. It is placed between the poles of a permanent magnet. You known that like poles repel each other and unlike poles attract. When the like poles of the electromagnet and permanent magnet are near, they repel each other and the armature turns. The commutator reverses the direction of the current and at the same time the poles of the electromagnet are reversed. The poles of the permanent magnet alternately repel and attract the electromagnetic poles and the armature rotates.