Saturday, April 26, 2014

De Magnete by William Gilbert

Apparently, no one had cut a magnet in half before.  Claims were made for the existence of gold magnets. Lodestone was a remedy for contrary medical symptoms.  Even the sailors who relied on lodestones wrongly called the poles of a magnet north and south for pointing north and south.  In 1600, while Galileo worked on mechanics in Padua, William Gilbert (or Gilberd) investigated magnetism in London. 

Galileo’s crime was questioning Aristotle.  Gilbert denounced Aristotle as a second-hander, a mere copyist for wives’ tales.  Galileo wrote in vernacular Italian. Gilbert wrote in Latin. Gilbert created new words for the new concepts he discovered: verticity for the tendency of an iron needle to point to a magnetic pole; versorium for an electroscope, i.e., a needle on a pivot to detect fields; coition for attraction because both bodies are mutually pulled to each other.  That last was a serious problem for the Victorian reader.  This Dover edition is a reprint of P. Fleury Mottelay’s 1889 translation. Mottelay nicely rendered this into an archaic kind of English, readable by moderns but spiced with older phrasings that perhaps more correctly delivered Gilbert’s own thinking.  Moreover, this edition is supported by copious footnotes from the author’s own research into the history of magnetics. 
You can find the original in Latin at the Lancaster University faculty projects archives here. Mottelay knows his science; and in translating, he does employ some modern terms, chief among them, "field."  In truth, Gilbert did not hypothesize fields. That thinking came later.

William Gilbert knew that the Earth is a magnet.  Earth’s magnetic field gives polarity to iron.  Heat a bar or needle of iron until it loses all attraction, then, place the bar aligned north and south and let it cool. It will acquire polarity. 

You can prove this for yourself, Gilbert enjoins you, by running the needle through a cork and floating it in a tub.  That is a simple apparatus; and he employed it over and over in different ways to tease out the facts about magnetism.

The beauty of this work is the intense and patient study behind it.  Gilbert was not publishing conjectures.  He was announcing empirical facts.  He called for experiments and observations in rejection of the compiling of authoritative citations from ancients.  In that, William Gilbert helped to nurture the Renaissance into the Age of Reason.   

He made some mistakes.  While clearly understanding that electricity is related to magnetism, he did not find evidence of electro-static repulsion.  He also claimed that magnetic variation is constant, though a generation later, it was measured as variable.  It is more important that he knew about variation, that the magnet aligns not quite true north-south, depending on the location on Earth.

Gilbert constructed models of the magnetic Earth, spheres of iron charged with polarity. He knew that bars and needles exhibit the phenomena better, but he had another point to make; and he did so repeatedly.  One of his spheres has a chunk missing. Another has a large protrusion. Thus, Gilbert demonstrated magnetic variation over the oceans and near mountains.



  1. Well-written and interesting review.

  2. There is a new improved English translation of William Gilbert's Latin 1600 De Magnete at