First, persuasion in  writing coincides with argumentation.  The purpose is to take a position on a topic/issue and with evidence or proof convince the audience to believe something. Aristotle wrote that all that was necessary to persuade was an  argumentative topic and evidence to prove it.  Today, persuasion includes both of Aristotle's elements with a third aspect: pathos or the emotions.

In persuasive writing, the same pattern follows: argument; proof; and emotions.  Voice, diction, tone, vocabulary choice, figurative language--these are the emotional choices made in  persuasive writing.  Joseph Conrad emphasized:

He who wants to persuade must trust not in just the right argument but in the right word.

That is when tone comes into play in the writing of a persuasive essay.

Tone in writing is defined as the attitude of the writer toward his subject.  The writer lacks the ability to show his facial expressions or body language, so he depends on his word choice to let the reader know where he stands on  a subject.  For example, in writing about television,  there is a big difference in these synonyms: the most informative invention in the 20th Century and the "boob tube."  Either way, the reader knows how the writer feels about television.

In addition, the tone has the ability to turn on or off the reader's agreement depending on the emotional weight of the writer's words. if the words are too strong, he may think that the writer is a fanatic and will not be persuaded.  Conversely, if the words are weak, the reader will not be believe the writer because he lacks authority.  The tone of the writer is presented through his word choices. The pen can paint what the eye cannot see. Furthermore, powerful words and the writer's tone can change the history of the the world. Remember the historical figure of the 20th Century who used these tools: Hitler in Nazi Germany.