These ball attachments will convert any Etalon micrometer, or other manufacturer's micrometer with the same spindle diameters, into an accurate tool for measuring tubing wall thickness. These will fit on either the anvil or the spindle side of the micrometer (your choice).
Because you will have added the dimension of the ball to the micrometer, you will have to subtract that diameter from any readings you take. Put a tag on your micrometer as a reminder to yourself or anyone else who might pick up the mike and try to use it.
Swiss made (as far as we can tell) rounded measuring faces are hardened and lapped steel. A dull chrome plated retainer fits over the anvils of your micrometers and holds it firmly in place.