Wiring question: house current to transformer

[nachoman]

I have some surplus small transformers (from a ceiling 12V halogen (?) light) that I want to use to power a throttle and other items.
I looked a Peter Thorne's book where he shows transformer wiring. His diagram shows the wiring in as having a fuse on one side and a switch on the other. Now, if I use a polarized plug, which side should get the switch and which the fuse?
What size fuse would I use on the 120V side if the output is rated 40W? -- Can you buy 1/3 amp fuses?

The 120 volt side is already protected by your house breaker. The lower voltage side is not protected against a short circuit if the fuse is on the high voltage side.