In optics we often talk of a ‘pencil beam’ of light, by which is meant a narrow beam of constant cross-sectional area. Such beams would obviously be drawn in diagrams using a pencil.
Writing implements have been around for a long time, whether simple reeds with which marks were impressed into wet clay tablets, or brushes of hair pushed into hollow plant fibres and used by the ancient Egyptians and Romans. It is the Latin for such brushes, penicillum, that has given us the modern word for pencil. One of the early implements simply used a metal stylus that left marks on many materials. A particularly useful metal for this was lead, and it is this association that has given us the term ‘lead’ of a pencil. In fact pencils haven’t used the element lead for a long time.
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