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XVIII. On the Production of Sudden Changes in the Torsion 

 of a Wire by Change of Temperature. By R. H. M. Bo- 

 SANQUET, St. John's College, Oxford*. 



[Plates VI. & VII.] 



THE wire which forms the subject of the following obser- 

 vations is the finest platinum wire which can be pro- 

 cured; its diameter is about '001 inch, and the length of the 

 piece used was about 3 feet 6 inches. It originally formed 

 the suspension of a galvanometer having a pair of nearly 

 astatic needles about 7 inches long. The steel needles could 

 be replaced by brass ones for the purpose of examining the 

 properties of the suspension. 



During the summer no disturbances worth attention were 

 noticed; but in the winter, when the use of the stove occa- 

 sioned rapid changes of temperature, disturbances became 

 evident. The brass needles were employed throughout the 

 present investigation. 



The tables and diagrams which follow, except the last, all 

 refer to one and the same state of the wire, the suspended 

 weight not having been lifted during their progress. The 

 lifting of the weight entirely changes the condition of the 

 wire so far as these phenomena are concerned. The last 

 table and diagram exhibit a subsequent state in which the 

 phenomena are less distinct and in reversed direction. 



The angular position of the needles was read on a scale of 

 degrees attached to the circular glass shade surrounding 

 them. This position, which we may call the elongation, is 

 the abscissa of the diagrams. The temperature was mea- 

 sured by a very sensitive thermometer with a long spiral 

 bulb, hung up alongside of the wire. The temperature in 

 degrees Fahrenheit forms the ordinate of the diagrams. 



The principal effects are as follows : — The elongation varies 

 between the limits of about 21° and 85°. Rising temperature 

 drives it up to about 85°, where it remains while the tempe- 

 rature continues to rise. Falling temperature drives it down 

 to about 21°, where it remains while the temperature con- 

 tinues to fall. The change of elongation is usually complete 

 or nearly so by the time that the reversed change of tempe- 

 rature amounts to from 2° to 5°. 



If the change of temperature is reversed as soon as the 

 elongation reaches either of its limits, the line of the diagram 

 returns along its original course : see diagram, April 27th. 



* Communicated by the Physical Society : read May 14, 1887. 



