May an abbreviation be used at the beginning of a sentence?

The preference is to spell out abbreviations that begin a sentence (except for abbreviated words that are never spelled out, like Mr. and Mrs.):

Electromotive force causes a greater quantity of electricity 

to flow through an electric circuit. 

not 

Emf causes a greater quantity of electricity to flow through an 

electric circuit. 

but 

Mr. Alfonso Guiterrez will assume the position of chief 

comptroller on Wednesday.