292 



Prof. J. J. Thomson. 



[June 20, 



From this we get the ratio of the torsional rigidity of the spider line 

 to that of the silk fibre to be 1 : 710. 



The diameters of the fibres were microscopically measured, and 

 gave the following values : — 



Silk fibre 0-00091 cm. 



Spider line , 0*00028 „ 



If the elastic qualities of these fibres were the same, the ratio of 

 the torsional rigidity would have come out (28) 4 : (91) 4 , or 1 : 112 ; 

 and hence the torsional rigidity of spider line is less than one-sixth 

 of that of silk fibre of the same thickness. 



The above result gives us only a relative value of the rigidities 

 between the two fibres. If we take the mean value of the torsional 

 rigidity of silk fibre to be 0'0012 C.GhS. on a length of 1 centimetre 

 (not per square centimetre), as found by Mr. T. Gray,* the tor- 

 sional rigidity of the spider fibre of the above experiment will be 



- — 0*000002 C.G.S., the mode of reckoning being the same. 



Mr. Gray's silk fibre may have had a slightly higher rigidity, as he 

 states that it was boiled in water, while the fibre of the experiment 

 just described was taken from those boiled in dilute potash water, as 

 is the usual practice of preparing " mawata," which is a very soft kind 

 of silk. 



IV. " Specific Inductive Capacity of Dielectrics when acted on 

 by very rapidly alternating Electric Forces." By J. J. 

 Thomson, M.A., F.R.S., Cavendish Professor of Physics, 

 Cambridge. Received June 17, 1889. 



The researches of Dr. John Hopkinson have shown that in some 

 dielectrics, of which the most conspicuous example is glass, the 

 refractive index is not, as it ought to be on Maxwell's theory, equal to 

 the square root of the specific inductive capacity, when the latter is 

 measured for steady forces, or such as are reversed only a few 

 thousand times a second. It is therefore desirable to measure the 

 inductive capacity under circumstances which approach as nearly as 

 possible to those which, according to Maxwell's theory, occur when 

 light passes through a dielectric. This will be when the forces are 

 reversed as rapidly as possible. In the following experiments the 

 forces were reversed about 25,000,000 times per second. 



The method consists in measuring the wave-length of the electrical 

 vibrations given out by a condenser whose plates are in electrical 

 connexion. If C is the capacity in electrostatic measure of the con- 

 * < Phil. Mag.,' 1887. 



