The vertical wires W^and W2 are freely hinged at the top and their lower ends dip into mercury pools. This leaves the wires to move sideways, but also, since mercury is a metal and conducts electricity, enables electric currents to be fed into the wires from the two storage batteries shown. When the storage batteries are connected so that the two currents flow in the same direction, thewires move toward one another. When the currents flow in the opposite direction, the wires moveapart. Thus parallel currents moving in the same direction attract each other. Parallel currents moving in opposite directions repel each other. It is convenient to imagine that one current produces some kind of field which exerts a force on the other current.
In the above experiment the wires always contain as many protons as electrons and do notaccumulate any net charge. The forces between the wires are not the electrostatic forces (Coulomb's Law), they are a consequence of the fact that electrons are moving inside the wires.Hence the field cannot be electrostatic. Moreover if we remove W2 and place a chargedconductor there, it experiences no force'. This field therefore, does not interact with stationary charges.
Further, if there is no current in the wire W2, the mutual force of attraction or repulsion also vanishes, although current in the wire W, and its field are still there. It follows,.therefore, that the field due to the current in the wire Wt acts on the wire W3 only when there is a current through it i.e., when there is a flow of charge through the wire W2.
From these observations from the above mentioned experiment, we come to the conclusion thatwhen two charged particles are both in motion, they exert on one another a new kind of force that depends upon their velocities and is zero if either velocity is zero. So, we have a field whichexerts, on a moving charge, a force, which is velocity dependent. We call it the magnetic field.This new field (or force) is responsible for the magnetic properties of materials such as iron bar magnets and also for the magnetic effects of electric currents.
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