62 Royal Society : — Mr. G. Gore on a momentary 



justed to the cohesive power of each metal ; if the stretching power 

 was too weak, the phenomenon sought for was not clearly deve- 

 loped ; and if too great, the wire was overstretched or broken when 

 it approached the softening-point. The amount of strain imparted 

 was approximately measured by temporarily substituting the body 

 of a small spring balance for the hooked wire F. The heated wire 

 must be protected from currents of cold air. 



With wires of iron 0'65 millimetre thick (size " No. 23") and 

 21*5 centimetres long, strained to the extent of 10 ounces or more, 

 and heated to full redness, the phenomenon was clearly developed. 

 As an example, the needle of the instrument went with regularity to 

 18*5 of index-plate ; the current was then stopped; the needle in- 

 stantly retreated to 17'75, then as quickly advanced to 19*75, and 

 then went slowly and regularly back, but not to zero. If the tempe- 

 rature of the wire was not sufficiently high, or the strain upon the 

 wire not enough, the needle went directly back without exhibiting the 

 momentary forward movement. The temperature and strain required 

 to be sufficient to actually stretch the wire somewhat at the higher 

 temperature. A higher temperature with a less degree of strain, 

 or a greater degree of strain with a somewhat lower temperature, 

 did not develope the phenomenon ; the wire was found to be per- 

 manently elongated on cooling. The amount of elongation of the 

 wire during the momentary molecular change was usually about -j^y 

 part of the length of the heated part of the wire ; but it varied in 

 different experiments ; it was greatest in amount when the maximum 

 degrees of strain were applied. The molecular change evidently 

 includes a diminution of cohesion at a particular temperature during 

 the process of cooling ; and it is interesting to notice that at the 

 same temperature during the heating-process no such loss of cohe- 

 sion (nor any increase of cohesion) takes place ; a certain tempera- 

 ture and strain are therefore not alone sufficient, to produce it ; the 

 condition of cooling must also be included. The phenomena which 

 occur during cooling are not the exact converse of those which take 

 place during heating. 



The phenomenon of elongation of iron wire during the process of 

 cooling evidently lies within very narrow limits ; it could only be 

 obtained (with the particular battery employed) with wires about 

 21*5 centimetres (=8^ inch) long, and about 0*65 millimetre 

 ( = Nos. 22 & 23 of ordinary wire-gauge) thick, having a strain 

 upon them of 10 ounces or upwards; with a weaker battery the 

 phenomenon could only be obtained by employing a shorter and 

 thinner wire. 



The experiment may easily be verified in a simpler manner by 

 stretching an iron wire about 1*0 millimetre diameter between two 

 fixed supports, keeping it in a sufficient and proper degree of ten- 

 sion by means of an elastic band, then heating it to full redness by 

 means of a row of Bunsen's burners, and, as soon as it has stretched 

 somewhat, suddenly cutting off the source of heat. In some experi- 

 ments of this kind, with a row (42 centimetres long) of 21 burners 

 and a row (76 centimetres long) of 43 burners, and the wire attached 



