In electromagnetism the types of matter are considered as conductor, insulator and semiconductor. An object is said to be a conductor if it permits the charge to move from one point to another within the material of the object. A conductor has considerable amount of free electrons which move when electric field is applied to it. For example, copper wire is a good conductor of electric charge.
The metals are always good conductor of electric charge. Now what's the case for an insulator? An object is said to be an insulator if it doesn't allow the movement of electric charge from one point to another within the material of the object. An insulator does not have free electrons or has very low free electrons. You can not find considerable amount of free electrons in an insulator. An example of insulator is a nylon thread.
And there is another type of material called semiconductor which has a property between the conductor and the insulator. This means a semiconductor has some free charges and allow the charges to move within the material. But a semiconductor does not allow the flow of charges as a conductor and does not stop the flow of charges as an insulator.
Now let's focus on charging by induction. First we consider a metal ball \(A\) which is initially positively charged hanging on a nylon thread. You can see Figure 2 for illustrative purposes. When you bring the charged metal ball near an uncharged metal ball \(B\) resting on an insulating stand as in Figure 2(a), negative charge is induced on the ball \(B\) on the side near the ball \(A\).
This happens because there is electrostatic attraction between opposite charges and repulsion between the same charges. The positively charged metal ball attracts electors towards it (because there are already free electrons in the uncharged metal ball) and pushes the positive charge away form it. Therefore, positive charge is induced on the far side of ball \(B\) until an equilibrium condition is reached; that is the attractive force between positive charge on ball \(A\) and negative induced charge on ball \(B\) is balanced by the attractive force between negative and positive induced charges on the ball \(B\).
The ball \(B\) is now connected to the ground by a conducting wire (see Figure 2(b)). Note that the Earth is a conductor and already has free electors and some of those free electrons flow through the wire to the ball \(B\) where the induced positive charge get those electrons and no positive charge remains on the ball \(B\) (but there is still negative induced charge until the ball \(A\) is placed near ball \(B\)).
When we first remove or disconnect the conducting wire from the ball \(B\) and later remove the ball \(A\), only a net negative charge remains on the ball \(B\). But if you remove the ball \(A\) first, the excess negative charge on the ball \(B\) (the charge that flowed to the ball from the ground) flows back to the ground through the conducting wire and the ball becomes electrically neutral again. In this way you can charge a metal ball by induction.
