Consult with the organisers before the start of the first session if there are any special announcements.
Warn the speakers that you will raise a finger/hand if they are over time and apologise for rudeness.
Introduce each speaker (name, university, title of their talk).
Keep track of time. Generally research talks should be divided into two parts of 25 minutes + 25 minutes with a brief break in between. If the speaker goes over time, politely raise a hand.
Start clapping at end. Ask the audience for questions. Thank speaker again at the very end.
If there are no questions, it is the chair's responsibility to ask something! (Anything!)
Organiser duties
In addition to the above the organisers should:
Come 20-30 minutes before to check: (i) visual tech works; (ii) audio tech works; (iii) recording tech works (Public Lectures); (iv) blackboard is clear and ready
Discuss the tech changeover with the chair and/or be on hand to assist with the technology
After the last speaker is done before lunch, ask audience to begin shuffling to lunch. After last speaker is done before end of the day, ask the audience to shuffle to the next location.