TRANSACTIONS OF THE SECTIONS, 41^ 



On the Thermostat, or Heat-governor, a self-acting physical 

 Apparatus for regulating Temperature. By Andrew 

 Ure, M.D., F.R.S., &c. 



This instrument acts by the unequal expansion of different 

 metals in combination : it admits of many modifications of ex- 

 ternal form, but, in all, the metallic bars must possess such force 

 of flexure in heating or cooling as to enable their working rods 

 or levers to open or shut valves, stopcocks, and ventilating 

 orifices. 



Steel and zinc are the two metals employed : they possess a 

 great difFei*ence of expansiveness, nearly as two to five, and 

 are sufficiently cheap to enter into the composition of ther- 

 mostatic apparatus ; but zinc has in reference to the present 

 object one property which should be corrected. After being 

 many times heated and cooled, a rod of that metal remains 

 permanently elongated. This property may, however, be in 

 a great measure destroyed, and considerable rigidity acquired 

 by alloying it with four or five per cent, of copper and one of 

 tin. Such an alloy is hard, close-grained, elastic, and very 

 expansible, and therefore suits pretty well for making the more 

 expansible bar of a thermostat. 



Let a bar of zinc or of this alloy be cast, about an inch in 

 breadth, one quarter of an inch thick, and two feet long, and 

 let it be firmly and closely riveted along its face to the face of 

 a similar bar of steel of about one third the thickness. The 

 product of the rigidity and strength of each bar should be 

 nearly the same, so that the texture of each may pretty equally 

 resist the strains of flexure. Having provided a dozen such 

 compound bars, let them be united in pairs by a hinge-joint at 

 each of their ends, having the steel bars inwards. At ordinary 

 temperatures the steel plates of such a pair of compound bars 

 will be parallel and nearly in contact, but when heated they 

 will bend outwards, receding from each other at their middle 

 parts, like two bows tied together at their ends. Supposing 

 this recession to be one inch for 180° Fahrenheit, then six such 

 pairs of bows, connected together in an open frame with 

 rabbeted end plates, and with a guide rod playing through a 

 hole in the centre of each, will produce an effective aggregate 

 motion of six inches, being half an inch for every 15° Fahr., 

 or 81" C. Instead of limiting ourselves to half a dozen such 

 pairs of compound bars, we may readily lodge in a slender iron 

 frame a score or two of them, so as to furnish as great a range 

 of motion as can be desired for most purposes of heat regu- 



o £ 2 



