Identify the labelled part responsible for opening and closing the contact breaker points in the provided diagram.

In the diagram:

• The "C.B. points" are the electrical contacts.
• The upper arm is the movable part of the contact breaker.
• The lower, rotating component with labels A, B, C, and D is the breaker cam.
• D is the center of rotation for the cam.
• A, B, and C are points on the profile of the cam.
• B and C represent the lobes of the cam, which are the raised sections.
• A represents a flatter section of the cam.

As the cam rotates, its lobes (like B and C) push against the rubbing block on the contact breaker arm, causing the points to open. When the flatter sections (like A) of the cam rotate past the rubbing block, the spring tension (not shown) pulls the points back together, causing them to close.

Therefore, the varying profile of the cam, specifically its lobes, causes the points to open and close. Among the labeled parts, B (or C) represents a lobe of the cam, which is the active part that pushes the points open.

The labelled part that causes the points to open and close is B.