What is the polarity induced at the ends A and B when the switch is pressed?
   

Hint: You can identify the polarity induced at the ends A and B in the given figure by applying right hand thumb rule. Therefore, get an idea about the right hand thumb rule and how it can be applied. So that you can identify the polarity induced at the given ends.

Complete step by step solution:
By observing the given figure, you can see the u-shaped soft iron bar AB. When the switch is kept pressed in on position, current starts flowing in the coils around the soft iron, so that the soft iron acts as an electromagnet.
We can say that there is an insulated copper wire which is wrapped on a soft iron piece. When the current is allowed through the coil, electrons flow through it. Current is passed using a battery and a key. Then the iron piece behaves and acts like a bar magnet until the current flow is stopped.
By using right hand thumb rule, we can say the polarity that is induced at the ends A and B when the switch is kept on is A is having south polarity whereas B is having north polarity.

Note: In the right hand thumb rule, we use the thumb, index and middle fingers at right angles to each other to find out the direction. The iron piece which behaves as a bar magnet when current is passed then is called an electromagnet.