I need to use the singular possessive form of 'apparatus'. Is it apparatus' or apparatus's?

Answer:
"At Langley, the preferred rules for forming possessives are as follows (G.P.O. 1984; and Rowland 1962):
• Form the possessive of a singular or plural noun not ending in s by adding 's.
• Form the possessive of a singular or plural noun ending in s by adding an apostrophe only"
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