28 IOWA ACADEMY OF SCIENCES. 



of science may be placsd in 1600, when Gilbert published his 

 "DeMagnete. " He was the first to use the terms magnetic 

 force, pole. He first studied the declination of the magnetic 

 needle, and first asserted that the earth is itself a great magnet. 

 Magnetic charts were made about the end of the century. 

 There is probably no branch of science that made such pro- 

 gress in the eighteenth century as electrostatics. It is very 

 largely a product of that century. The frictional electric 

 machine of Von Guericke of the seventeenth century consisting 

 of a revolviag ball of sulphur rubbed by the hands, was gradu- 

 ally improved by the substitution of a glass globe, then a glass 

 cylinder, and finally a glass plate for the sulphur ball, and fixed 

 pads with amalgam fiaally took the place of the hands. 



Stephen Gray electrified the human body in 1730. Du Fay 

 repeated Gray's experiment and finally arrived at the conclu- 

 sion that all bodies may be electrified, and he discovered that 

 there are two kicds of electticity. The Leyden jar came in 

 1745-46 and created no end of interest. There followed soon 

 the ideas of insulation, induction, and potential. The influence 

 of points in dissipating the electric charge was dwelt upon by 

 Franklin, who studied atmospheric electricity, arriving at the 

 identity of electricity and lightning, and suggested the protec- 

 tion of buildings by pointed rods. 



In 1747 Franklin advocated a one fluid theory. A body hav- 

 ing a certain charge was neutral; less than this amount gave 

 the effects called negative; more gave the effects associated 

 with p sitive electricity. 



The litter half of the centary saw great; advances in electric 

 measurements. Cavendish, the recluse, studied electrical 

 measurement and bsfore 1771 arrived at clear conceptions of 

 inductive capacity and constructed a set of condensers, and 

 determined the capacity of several substances. He proved 

 that static charges are on the outside of hollow bodies, that 

 electric force varies inversely as the square of the distance, but 

 his writings were not made public for a century. 



Coulomb inven'ed the torsion balance, and proved that the 

 force of electric attraction or repulsion varies inversely as the 

 square of the distance, and as the product of the quantities of 

 the electricities. He showed that electric charges reside on 

 the surface of bodies, and revived the two fluid theory of Du 

 Fay. 



