18 



HEAT. 



arm on the scale being noted. But there are two serious objections to 

 this apparatus. Firstly, it is difficult to maintain the bar at any desired 

 temperature ; and secondly, during the experiment the measuring part 

 of the apparatus may change in dimensions, as well as the body to be 

 measured. For accuracy it is necessary that the measuring apparatus 

 shall remain absolutely at a constant temperature, while the body to 

 be measured shall be varied in temperature in a known manner. The 

 latter condition may be fulfilled by taking the length of the body first 

 in melting ice, then in boiling water. To fulfil the former condition 

 several methods have been adopted. 



Ramsden's Method. One of the best is that of Ramsden, devised 

 in 1785 to determine the expansion of the rods used by General Roy to 

 measure the base line on Hounslow Heath, on which was founded the 



original Ordnance 



Survey of the 

 United Kingdom. 

 The apparatus, 

 of which the gene- 

 ral arrangement 

 may be gathered 

 from Fig. 10 and 

 the accompanying 

 description, con- 

 sisted of three 

 troughs, each over 

 5 feet long, placed 



FlO. 10. Plan of Ramsden's Expansion Apparatus. (1) Wood parallel on a table, 

 trough at containing cast-iron bar bb, with cross wires Cast-iron bars were 

 in uprights uu' fixed near ends ; uu' shown also in eleva- fixed in the two 

 tion; (2) copper trough with lamps underneath to raise i v," v, 



temperature, and containing bar BB to be tested, upright e a Wougns, wmcn 

 U pressed against left end by adjusting screws SS, upright were always filled 

 U' pressed against right end by spring sp. UU' carry with melting ice, 

 microscope object-glasses; (3) wood trough at contain- so that the bars 

 ing cast-iron bar b'V with uprights w/ fixed near ends , : nvar : n ui ft 



carrying microscope eyepieces, that on v' being provided * 

 with a micrometer m, length. Near the 



ends of the bar, bb y 



were fixed uprights carrying cross wires. In the middle trough were 

 two sliders, moving only horizontally along the trough with uprights 

 UU' at their more distant ends, carrying the object-glasses of two 

 microscopes. The bar to be tested was placed on rollers in this trough, 

 and its ends were made to bear always against the uprights UU'. 

 The upright U was kept fixed so that any expansion pushed out the 

 upright U'. Near the ends of the bar b'b' were fixed uprights, carrying 

 the eyepieces of the microscopes, each with a vertical cross wire, the wire 

 at v being fixed, that at v' being movable by a micrometer screw m. The 

 middle trough could be heated by means of lamps. 



The general nature of an experiment was as follows : All three 

 troughs being filled with melting ice, the uprights were so adjusted that 

 the cross wires on uu', seen in the microscopes, were in the centre of the 

 field, and coincided with the eyepiece wires. The middle trough was 

 then filled with hot water, which was further heated by the lamps to 



