On the Effects of Changes of Temperature on Metal Wires. 185 



ing jacket. The temperature of the internal space was observed before 

 and after each series of readings, and in most cases also while the series 

 of operations was in progress. 



The distance between the needles was measured at the lower tem- 

 perature ; the distance at the higher temperature was obtained with 

 sufficient accuracy from the difference of temperature and the result of 

 a determination specially made of the coefficient of linear expansion 

 of the wire. This coefficient was determined during the heating of the 

 wire, and, as the change of length was beyond the range of the micro- 

 scope scale, a scale was fixed behind the lower needle, and the increase 

 of length read off on it by means of a telescope. The radius of the 

 wire was determined after the wire was finally taken down by means 

 of a micrometer from twenty readings taken at different points by each 

 of two observers. 



In two cases (wires II and III) the radius was determined by careful 

 weighing of the wire in air and in water, and the results so obtained, 

 properly corrected, agreed with those got by the micrometer method to 

 within T a ff per cent. 



Experiments were made for the following wires : I, German silver ; 

 II, mild steel ; III, brass ; IV, copper (commercial) ; V, copper (hard 

 drawn electrolytic) ; VI, soft iron. At first everything did not work 

 quite smoothly, the arrangement of the microscopes required a little 

 adjustment, and the determinations of moduli for I were somewhat 

 doubtful in point of accuracy, and for II were certainly in error. The 

 apparatus was, however, got into thorough order before the experi- 

 ments on III were begun,' and those on II were thereafter repeated. 

 The following is the table of results for Young's modulus, in which is 

 included the value obtained for German silver, notwithstanding the 

 doubt as to its correctness, and the later value for mild steel, since 

 there were good grounds for rejecting that obtained at first. The radii 

 given in column VI were calculated for the higher temperatures from 

 the expansions observed. The results of three separate determinations 

 are given for the wire of soft iron (VI). Each elongation was obtained 

 from the mean of about ten separate observations, agreeing to less than 

 0*5 of a microscope division. The elongations were obtained in micro- 

 scope divisions, the value of which was 0-005948 cm. 



The difference between the second and third values of the coefficient 

 of diminution for soft iron and the first of the three values, depends 

 upon O2 of a microscope division. The degree of accuracy to which 

 the deflections could be read was to 0-1 of a division. The lower 

 value, 0-000136, is probably more nearly correct than the higher, 

 0-000197. 



Comparing the results with those obtained by Mr. Shakespear, re- 

 duced for comparison, we have for steel 0'000247 as against his 

 0-00038, for brass 0-000373 as against 0-000352, for copper (electro- 



