544 Mr. G. A. Shakespear on an Interference- Method for 



apparatus, by means of a central bolt. After this the vibra- 

 tions seemed as bad as when the apparatus was suspended 

 from a wall-bracket, but the addition of two half-hundred- 

 weights to the stand, and supporting the weight-tank on 

 indiarubber blocks, improved matters, and the further 

 addition to the lower stretching-disk of three vanes which 

 dipped into dash-pots of oil practically removed the trouble, 

 and now, although heavy traffic in the street occasionally 

 causes disturbances, vibrations are so rapidly damped as not 

 to interfere with work. 



The apparatus was first used to test the modulus of some 

 copper wire (\ inch diam.) specially drawn straight (by Messrs. 

 Glover of . Salford), and loaded with varying initial loads, 

 beginning with an initial load of 2791 grins, for the three 

 wires — i. e. about T Jy of the estimated breaking-weight — and 

 proceeding with successive additions of 2 kilos. About twenty 

 observations were made at each stage, the weight producing 

 the extension observed being always the same. It may be 

 remarked here that the stretching-weight above described was 

 standardized by weighing the cylinder directly (on Prof. 

 Poynting's large balance which was kindly put at my disposal) 

 when the water was respectively at the upper and lower levels 

 above mentioned, the temperature of the water being noted. 

 The difference between these weights (at 15°C.) was 5989*50 

 grms. 



The result of this set of observations is indicated in the 

 curve (rig. 6), in which the modulus is the ordinate and the 

 initial load the abscissa. The modulus began at 8'24x 10 11 

 (c. G. s.) and gradually increased for successive increments of 

 initial load until for an initial load of -fa- of breaking- weight 

 it reached the value 1*053 x 10 12 , after which it seitled down 

 and the curve became sensibly parallel to the load axis as far 

 as the experiments went. On reducing the initial loads again 

 the curve practically went back the same way as it came. 

 Perhaps experiments of this kind may throw light on the 

 discrepancies found by Wertheim between values of the 

 modulus obtained by different methods. It may be pointed out 

 that if the wires were slightly curved, some such effect as that 

 just described might be expected. 



Heating. — This was a matter of some difficulty. Stirring- 

 was inadmissible on account of vibrations, hence water 

 could not be used. Heating by means of a current of 

 electricity through the wires themselves was out of the ques- 

 tion ; for (1) the current itself apparently affects the elasticity, 

 and (2) the heating would tend to be patchy owing to 

 differences in radiating power of different parts of the surface 



