The part of a bucket's boom tip apparatus that is electrically tested is the ___.

The bucket liner is the correct answer because it serves as an essential component in ensuring the safety and effective operation of the boom tip apparatus. The bucket liner is typically part of the inner structure of the bucket designed to protect it from wear and damage, as well as to insulate against electrical hazards.

Electrical testing is particularly important for components like the bucket liner, as it allows for the identification of any vulnerabilities that could lead to electrical failures during operation. This testing helps ensure that there are no conductive pathways that could pose risks to operators or equipment.

The other options, while integral to the structure or operation of the bucket, do not specifically relate to the electrical testing aspect that is highlighted in the question. For instance, the outside bucket shell primarily provides physical protection to the bucket but is not part of the electrical testing protocol. The boom tip control is related to the operational controls of the boom but does not encompass the testing aspect, and the material jib is more about the mechanical support rather than electrical properties. Therefore, the focus on the bucket liner underscores the critical aspect of electrical safety in such equipment.