82 Biographical Sketch of [August, 



topics which he discusses in this valuable memoir is that of 

 friction ; he examined the opinions of those who had already 

 treated upon it ; he repeated and varied their experiments ; and 

 proceeding upon a larger scale, he obtained results which were 

 in many respects novel, and altogether very interesting. Some 

 of the most curious observations which he made were respecting 

 the relation between the length of time in which the effect of 

 friction reaches its maximum quantity, and the amount of the 

 weight or force employed. This relation he found to be of the 

 greatest importance in a practical point of view, and to influence 

 the results so materially, that unless it is taken into account, all 

 our calculations must be fundamentally erroneous. For example, 

 supposing that the force required to overcome the friction of one 

 surface upon another, as depending upon a certain pressure on 

 the surface, when the bodies were first placed in contact, was 

 100, in a few seconds it would be as 250 or 300, and in a few 

 days it would increase to 900 or 1000.* 



In the researches to which he was led in his experiments on 

 the construction of the compass, he had occasion to pay parti- 

 cular attention to the effects of what he stiles torsion, or the 

 resistance which the suspending wire opposes to the action of 

 the needle, in obeying the magnetic attraction. This circum- 

 stance was the cause of Coulomb's invention of what he deno- 

 minated his torsion balance, an instrument which he afterwards 

 employed very extensively for measuring minute forces, such as 

 those produced by extremely small quantities of electricity and 

 magnetism. An account of his experiments on this subject was 

 published in the memoirs of the Academy of Sciences for 1784, 

 under the title of theoretical and experimental researches on the 

 force of torsion, and the elasticity of metallic threads. The action 

 of the torsion balance essentially consists in the resistance which 

 an extremely fine thread opposes to our attempts to twist it, and 

 his object was to obtain an accurate measure of the force of this 

 resistance. The nature and construction of the instrument are 

 too well known to require any minute description ; it may be 

 stated, in general terms, as consisting of a metallic wire, which 

 is fixed at its upper end and is suspended in a vertical direction, 

 while to its lower end is attached a cylinder connected with a 

 horizontal index ; by causing the arms of the cylinder to revolve 

 upon the point of suspension, the wire to which it is attached 

 is twisted ; and when we cease to twist it, its elasticity causes it 

 to assume its natural position. The index in this case will pass 

 through a certain space which is measured by a graduated scale.f 

 Coulpmb's experiments led him to conclude that the force with 



• Mem. Syav. Etrang. x- 163. The prize was awarded in 1781, and the paper 

 printed in 1784. 



+ A figure of the torsion balance may be found in the Supplement to the Enc. 

 Brit. PI. 27, figures 1, 2, 3, 4, 5, also in Dr. Brewster's Encyc. Art. Electricity, 

 PI, 244, fig. 7, 8, 9, 10. 



