ISO GREAT MEN OF SCIENCE 



that these forces are always proportional to the quantities 

 acting upon one another,^ and inversely proportional to the 

 square of their distance apart. Coulomb's law for magne- 

 tism is also exactly similar. Both laws are thus completely 

 analogous to Newton's law of gravitation discovered one 

 hundred years previously. In spite of their simplicity and 

 similarity with what has long been known, the recognition 

 and indubitable proof of these laws was nevertheless at that 

 time a rare masterpiece of experimental skill, and more than 

 this, it required means to be found by which the peculiar 

 electric and magnetic phenomena could be grasped quanti- 

 tatively, in order to bring to bear on them Pythagoras' 

 ancient insight - the power of number. The forces to be 

 measured are very small, and they are very fleeting, for 

 electricity itself, as Coulomb himself found, and also 

 observed quantitatively, even dissipates itself into the air. 

 For measuring small forces. Coulomb worked out the 

 special method of the torsion balance, which has since been 

 made use of in a large number of the finest measuring 

 instruments (galvanometers, electrometers, etc.), all of which 

 depend upon allowing the forces to be measured to twist a 

 very thin fibre or wire, upon which they act through the 

 arm of a lever, suspended in a horizontal position by the 

 fibre or wire. The amount of the resulting angle of twist, 

 at which the force to be measured is in equilibrium with the 

 opposing elastic force of the fibre, is then a measure of the 

 force. Coulomb was thus obliged to examine the laws of 

 the forces required to twist fibres and wires. He found them 

 to be proportional to the angle of twist, and also to the fourth 

 power of the diameter of the wire, inversely proportional to 



^ The proportionality of quantity was not proved by Coulomb by 

 means of special experiments, since he takes the forces from the start as a 

 measure of the quantities of the unknown electricities and magnetisms, 

 but not without having previously proved that this assumption can be 

 carried out consistently, by numerous experiments, for example on the 

 division of quantities of electricity between conductors brought in con- 

 tact with one another. 



