126 



Domestic Science 



The straw will slowly revolve in the opposite direction 

 to that previously described, and will at length come 

 to rest in an approximately vertical position, with the 

 same end at the top as was the case when the experiment 

 was started. ,* 



Since the straw cannot slip round the needle, its 

 motion must be caused by the rotation of the needle 

 itself. This rotation is caused by the elongation of the 

 poker under the influence of the heat supplied by the 

 burners. The amount of lengthening is small and would 

 be overlooked if it were not multiplied in some way, as 

 it is in this experiment by the straw-needle combination. 



If a bar of equal length to that used above, but 

 made of another metal, such as brass, be substituted 

 for the iron bar, similar observations will be made. 

 If careful note be taken, however, of the number of 

 revolutions of the straw in each case, these numbers 

 will prove to be quite different for different metals. 

 This indicates that different solids expand to different 

 extents when heated simi- 

 larly. For instance, the brass 

 rod will cause the straw to 

 make about 1 J times as many 

 revolutions as a rod made of 

 iron. 



82. That the effect of 

 heating a solid body is not 

 merely to increase its length, 

 but that it is also increased 

 in bulk by being heated, may 

 be shown by the following 

 experiment : Fig. 35. 



EXPERIMENT 40. The apparatus required is 

 generally known as Gravesande's Ball and Ring. 



