56 Journal of the Asiatic Society of Bengal, [February, 1914.] 



In using the apparatus for experiments with a long wire, 

 the magnification can be diminished by making the length of 

 the rod between P and the edge of the slit t (Fig. I) sufficiently 

 long so that the spot of light may remain on the scale for the 

 greatest elongation to be plotted. Figures III and IV are the 

 curves for brass and steel respectively. They are plotted by 

 observations on lengths of over 3 metres of wire, the magnifi- 

 cation of the instrument in each case beinc 78. 



Length of brass wire 333 cms. Diameter of the wire 

 •045 cm. From Fig. Ill it is found that when the load is 

 equal to COO grams the elongation is 9'4/78 cm., whence 

 the value of Young's modulus for the specimen is found to 

 be 10-2 x JO 11 dynes cm/ 2 The value obtained by the ordi- 

 nary laboratory method was 10*61 x 10 11 dynes cm.* 



h of steel wire used 333 cms. Diameter '034 cm. 

 From the curve when the load is 600 grams the elongation 

 is found to be 8*4/78 cm., whence the value of Young's 

 modulus is found to be 20 x 10 11 dynes/cm. 4 The value 

 obtained by the ordinary laboratory method was 19*9 x 

 10". 



This apparatus is described not because its value as an 

 accurate scientific instrument is emphasized, but because it 

 forms a very convenient arrangement for demonstration pur- 

 poses and is believed to supply what has long been wanted 

 among lecturers in Elementary Physics. Judging from the 

 degree of accuracy which it affords in determining Young's 

 modulus it appears that its use may not be altogether pre- 

 cluded in the laboratory, especially for the investigation of 

 those portions of the load-elongation curve for the soft metals, 

 which are beyond the elastic limit. 



In conclusion, I take this opportunity of expressing my 

 gratitude to Professor E. P. Harrison who has given me the 

 necessary facilities in the Physical Laboratory of Presidency 

 College and has encouraged me with his valuable advice. 



r \ 



f\* 



