QUAETZ FIBEES* 



By 0. V. Boys, F. R. S. 



lu almost all investigations which the physicist carries out in the lab- 

 oratory, he has to deal with and to measure with accuracy those subtle 

 and to our senses inappreciable forces to which the so-called laws of 

 nature give rise. Whether he is observing by an electrometer the be- 

 havior of electricity at rest, or by a galvanometer the action of elec- 

 tricity in motion ; whether in the tube of Crookes he is investigating the 

 power of radiant matter, or with the famous experiment of Cavendish 

 he is finding the mass of the earth — in these and in a host of other cases 

 he is bound to measure with certainty and accuracy forces so small that 

 in no ordinary way could their existence be detected; while disturbing- 

 causes which might seem to be of no particular consequence must be 

 eliminated if his experiments are to have any value. It is not too 

 much to say that the very existence of the physicist depends upon the 

 power which he iwssesses of producing at will and by artificial means 

 forces against which he balances those that he wishes to measure. 



I had better jierhiips at once indicate in a general way the magnitude 

 of the forces with which we have to deal. 



The weight of a single grain is not to our senses appreciable, while 

 the weight of a ton is sufficient to crush the life out of anyone in a 

 moment. A ton is about 15,000,000 grains. It is quite possible to 

 measure with unfailing accuracy forces which bear the same relation to 

 the weight of a grain that a grain bears to a ton. 



To show how the torsion of wires or threads is made use of in meas- 

 uring forces, I have arranged what I can hardly dignify by the name of 

 an experiment. It is simply a straw hung horizontally by a piece of 

 wire. Resting on the straw is a fragment of sheet-iron weighing 10 

 grains. A magnet so weak that it can not lift the iron yet is able to 

 pull the straw round through an angle so great that the existence of the 

 feeble attraction is evident to everyone in the room. 



Now it is clear that if, instead of a straw moving over the table simply, 



'Lecture delivered at tLe Royal Institution, on Friday, Jnue 14, 1P89. (From Natm'e, 

 July 11, 1889, and October 16, 1890, vols. XL, pp. 247-251, and xlii, pp. 604-608.) 



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