Is it a uniform or an uniform?

Question

On a Physics specification, it says:

6.7 Know how to use two permanent magnets to produce a uniform magnetic field pattern.

Isn't it "produce an uniform magnetic field", or is the existing "produce a uniform magnetic field pattern" correct?

Answer 1

Whether you use a or an is entirely decided by the sound of the following word, and doesn't necessarily have anything to do with the spelling of the following word. Words like uniform or user are pronounced with an initial "y" sound (a palatal glide), and so it's correct to say a uniform, a user. Saying an uniform, an user is incorrect.

Answer 2

The rule of placing an before a vowel is actually to place an before a vowel sound. Therefore, because uniform is pronounced with a "y" sound (/ˈjuːnɪfɔːm/), which is not a vowel sound, it takes an a before it.