Is Water Necessary for Contact Electrification?

Was Thales right about water? Contrary to previous reports, contact charging can occur in the absence of water. At the same time, water helps stabilize the developed charges. Water-free conditions are realized by performing all experiments and charge measurements under oil-immersion. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.