3.1 Introduction

The electrostatic and magnetic phenomena were observed even in BC. As early as 600 BC, Thales a Greek philosopher observed the attracting properties of amber. The attracting properties of certain stones in magnesia were observed in 800 BC.

Both electricity and magnetism has been known for more than 2000 years. However it was only about 200 years ago,in 1820, that it was realized that they were intimately related.During a lecture demonstration in the summer of 1820, the Danish Physicist Hans Christian Oersted (Hans Christian Oersted (1777-1851) was a Danish physicist and chemist, professor at Copenhagen. He observed that a compass needle suffers a deflection when placed near a wire carrying an electric current. This discovery gave the first empirical evidence of a connection between electric and magnetic phenomena ) noticed that a current in a straight wire caused noticeable deflection in a nearby magnetic compass needle. He investigated this phenomenon. He found that the alignment of the needle is tangential to an imaginary circle which has the straight wire as the center and has its plane perpendicular to the wire. It is noticeable when the current is large and the needle sufficiently close to the wire so that the earth's magnetic field may be ignored. Reversing the direction of the current reverses the orientation of the needle. The deflection increases on increasing the current or bringing the needle closer to the wire.

The science of magnetism blossomed with the publication of the famous book De Magnete in 1600 by William Gilbert. The science of electricity started to grow from the time of Benjamin Franklin. For a long time it was believed that electricity and magnetism are independent of each other

Iron filings sprinkled around the wire arranged themselves in concentric circles with the wire as the center. Oersted concluded that moving charges or currents produced a magnetic field in the surrounding space. Following this there was intense experimentation. In 1864, the laws obeyed by electricity and magnetism were unified and formulated by James Maxwell who then realized that light was electromagnetic waves. Radio waves were discovered by Hertz, and produced by J.C. Bose and G. Marconi by the end of the 19 th   century. A remarkable scientific and technological progress has taken place in the 20 th century. This is due to our increased understanding of electromagnetism and the invention of devices for production, amplification, transmission and detection of electromagnetic waves.

In 1820 in a lecture demonstration Hans Christian Oersted noticed that current in a straight wire caused a noticeable deflection in a nearby compass needle. He also found that on reversing the direction of current the direction of deflection is also reversed. On further experimental studies he found that deflection increases on increasing the current or in bringing the magnetic needle closer to the wire.Finally Oersted concluded that a moving charge or current produces a magnetic field in surrounding space.

In this chapter we will see how magnetic field exerts forces on moving charged particles, like electrons, protons and current carrying wires. We shall also learn how currents produce magnetic fields. We shall see how particles can be accelerated to very high energies in a cyclotron. We shall study how currents and voltages are detected by a galvanometer.

Oersted's discovery revolutionized the science of magnetism and electricity. By 1820s the scientific world realized the fact that electricity and magnetism are interrelated phenomena and the cause of both effect are basically electric charges and their motion.

In the subsequent chapters on magnetism, we adopt the following convention: A current or a field (electric or magnetic ) emerging out of the plane of the paper is depicted as a dot. A current or a field going into the plane of the paper is depicted by a cross.

Cyclotron

Biot-Savart's Law