500 



Mr. S. J. Loclmer on the Elongation 



I made some experiments with my instrument to ascertain 

 whether any uncertainties entered into my observations from 

 this cause. I found that when the centre of the bar fell even two 

 or three centimetres from the centre of the coil, the measure- 

 ments were the same as when it was centred as nearly as 

 possible. When, however, the bar was thrown completely off 

 from the centre, as much, for instance, as 10 or 15 centimetres, 

 I found a slight but measurable effect amounting to about 0*2 

 of a fringe, or "000002 of an inch. 



Early in the experiment I found that for a given magnetic 

 field I could produce different amounts of expansion depending 

 upon the manner in which the current had been applied — for 

 instance, whether the full amount of the current had been 

 turned on suddenly or, on the other hand, had been applied 

 gradually, then again whether the current had been gradually 

 increased from zero or had been decreased from a higher 

 point. 



Let me here mention that it was observed that the second 

 and following contacts did not give the same expansion as the 

 first contact ; this has already been well shown by Prof. A. 

 M. Mayer's observations. 



In the following table the values dealing with the second 

 and following contacts show some of these peculiarities quite 

 conclusively. Column I. gives the amount of expansion in 

 fringes, column II. the corresponding current when it was 

 suddenly turned on at its full strength, column III. when 

 gradually increased, and column IV. when gradually de- 

 creased from the current which produced the maximum ex- 

 pansion. 



Bar No. I. was here used All currents were measured with 

 a direct-reading ammeter. 



I. 



II. 



HI. 



IV. 



1 



0-22 



0-22 





2 



0-27 



0-27 





3 



033 



0-30 





4 



0-37 



0-33 





5 



0-46 



0-38 



0-18 



6 



0-49 



0-44 



0-22 



7 



0-56 



0-51 



0-25 



8 



0-66 



0-57 



030 



9 



084 



0-75 



O-40 



10 



117 



102 



055 



10-3 



1-84 



1-72 



067 



11-0 







100 



