Good afternoon , everyone . The experiment is to start at 3 . Have you gone through the instructions ?   Yes , sir .   Ok , now some points for attention . First of all , pay attention to safety ... Now , sign your names on the lab record , and after that you can start . If there is any question , just let me know .   Excuse me , sir . This air compressor doesn't work .   Have you turned it on ?   Yes , I have .   Ok , I will get you another one . 