30 HISTORY OF ELECTRICITY. 



strictly 2, but to leave it indeterminate between 1 

 and 3; but in his applications of his results, he 

 obviously inclines to the assumption that it is 2. 

 Experimenters tried to establish this in various 

 ways. Robison 9 , in 1769, had already proved that 

 the law of force is very nearly or exactly the inverse 

 square; and Mayer 10 had discovered, but not pub- 

 lished, the same result. The clear and satisfactory 

 establishment of this truth is due to Coulomb, and 

 was one of the first steps in his important series of 

 researches on this subject. In his first paper 11 in 

 the Memoirs of the Academy for 1785, he proves 

 this law for small globes ; in his second Memoir he 

 shows it to be true for globes one and two feet in 

 diameter. His invention of the torsion-balance, 

 which measures very small forces with great cer- 

 tainty and exactness, enabled him to set this ques- 

 tion at rest for ever. 



The law of force being determined for the par- 

 ticles of the electric fluids, it now came to be the 

 business of the experimenter and the mathema- 

 tician to compare the results of the theory in detail 

 with those of experimental measures. Coulomb 

 undertook both portions of the task. He examined 

 the electricity of portions of bodies by means of 

 a little disk (his tangent plane] which he applied 

 to them and then removed, and which thus acted 

 as a sort of electric taster. His numerical results, 



9 Works, iv. p. 68. 10 Biog. Univ. art. Coulomb, by Biot. 



11 Mem. A. P. 1785, pp. 569, 578. 



