Question

Comparing two cases, is it both cases are the same type? Would you expect to see the needle move the direction it did?:

Case 1: Charge up the rubber rod with the wool cloth. Transfer a small amount of the charges from the rubber rod to the plate atop the electrometer.

Cases 2: Charge up he glass rod with silk cloth. Bring the charged glass rod near the electrometer plate with out touching it.

Answer 1

Case 1:

In first case when rubber rod is rubbed with a wool cloth,the wool cloth gets positively charged by losing an electron to the rubber rod while rubber rod gains an electron and becomes negatively charged.

When this negatively charged rubber rod placed near the electrometer, it exerts a electrostatic coulomb force of attraction to the positive charged needle of electrometer.

Case 2:

In second case when glass rod is rubbed with a silk cloth,the glass rod gets positively charged by losing an electron to the silk cloth while silk cloth gains an electron and becomes negatively charged.

When this positively charged glass rod placed near the electrometer, it exerts a electrostatic coulomb force of repulsion to the positive charged needle of electrometer.

Conclusion :

In both these cases the direction of deflection of needle is opposite to each other.