Which is the rule for the pronunciation of the possessive when a proper noun ends in s?

Question

In "Weiss's thermal drill" we say "Weisses thermal drill" whereas in "James' house" we do not say "Jameses house".

Answer 1

In modern, British English we would write "James's house" and say James-iz.

It is only ancient/biblical names that, when ending in "s", do not take the possessive "s" ending, thus: "Jesus' disciples" is pronounced Jesus disciples.

My name ends in "s" and I teach English so I should know!

Answer 2

I'm guessing the second s at the end of Weiss is what changes the convention

Answer 3

Fidget's right!  Get "The Elements of Style" by Strunk and White.  It's chapter one, page one, rule one:

Form the possessive singular of nouns by adding 's.  Exceptions are the possessives of ancient proper names in -es and -is (Jesus', Isis', Moses').

For a name like Weiss or James, the possessive is with an 's and they're pronounced Weisses and Jameses.