1. Paper is a poor conductor of magnetism, so magnetism exists in books for a long time.

2. When the degaussing book passes through the sensor, the magnetism in the book will produce electromagnetic effect on the wires in the sensor, thus giving an alarm.

3. When degaussing, you can see that the salesperson puts the book on a plastic box, which is degaussing.

There is a magnetic pole opposite to that in the book in the plastic box, because the magnetism in the book is caused by the friction between the book and the magnetic pole.

If there is only one magnetic pole, the magnet in the plastic box is opposite to the magnetic pole in the book, and the two cancel each other out, so it will be demagnetized.