BREGUET'S THERMOMETER. 



265 



Obtain two strips of sheet copper and iron respectively, some two 

 inches wide and two feet long, of pretty stout material ; have them 

 firmly riveted together so as to form a flat double bar; on 

 heating this with a lamp the bar will curve, because, as the copper 

 expands more than the iron, it must either break the rivets or 

 else take the outside of a curve so as to occupy a greater length 

 than the less expanded iron on the inside of the curve. On cool- 

 ing down the curved compound bar will gradually become flat 

 again ; if chilled in a freezing mixture the bar will again curve, 

 but now with the copper inside the curve, since the copper contracts 

 the most. In laying railroads, building iron joists into walls, and 

 such like operations, the differences in length caused by the varia- 

 tions of temperature between day and night, summer and winter, 

 &c., have to be 



carefully allowed 

 for, and space left 

 to permit of ex- 

 pansion; were this 

 not done the rails 

 would become 

 curved, the walls 

 thrust out of 

 the perpendicular, 

 and similar seri- 

 ous derangements 

 brought about. A 

 long iron girder 

 bridge will often 

 become measur- 

 ably longer if a 

 gleam of sunshine 

 fall upon it on a 

 cloudy day, or 

 shorter if a cool 

 wind suddenly Fi 8- 120 ' Breguet's Thermometer, 



spring up and blow upon it ; with some structures the amount of 

 growth and decrease can be readily watched at the ends by the 

 eye under favourable circumstances. 



Breguet's metallic thermometer (fig. 120) is an instrument 

 depending on the different rates of expansion of different metals. 

 A long compound spiral, HH, is formed of two or more metals with 

 the most expansible one inside ; usually a triple strip is employed 

 with silver inmost, gold next, and platinum outside. As the tem- 

 perature rises, the silver expands most and the platinum least, so 



