Explanation:

- The thermometer is an instrument for measuring temperature.

- The glass form consists of a glass tube with a fine bore.

- One end of the tube is blown to form a bulb and the other end is closed.

Options explanation:

- Option:1, tube: Correct. The tube is the part that is expanded to form the bulb.

- Option:2, edge: Incorrect. The term ‘edge’ is not relevant to this context.

- Option:3, centre: Incorrect. The centre is not involved in forming the bulb.

- Option:4, place: Incorrect. The word 'place' is irrelevant in this context.