436 Mr. J. Hopps on the Alterations in the 



same temperature as on the previous evening, and kept con- 

 stant within half a degree Fahr. + . 



In the case of some wires as many as 380 readings have 

 been taken altogether ; this was on lead wire No. 5. At the 

 end of the seventh series the wire broke. 



The curves of resistance of four of the seven series of tests 

 on this wire is very interesting. It will be observed that, while 

 being very similar to each other, there is a point marked (a) 

 on each series where coiling and uncoiling produce an increase 

 in the resistance. Again, the wonderful power of recovery 

 which is shown by the wire after each rest is very marked ; 

 the normal condition would appear to be to give positive and 

 negative readings, and that this condition is upset after a 

 time. The inability to recover this condition has suggested 

 that, when this point has been arrived at, it might be said to 

 represent the commencement of a kind of electrical fatigue. 



There are some other points on this diagram, and which 

 are more or less repeated by every other wire that I have 

 tested, which furnish matter for speculation ; such as — why 

 should coiling generally produce a less effect than uncoiling ? 

 and why should the signs be generally different? and why 

 should those signs in many cases become reversed ? 



An increase of resistance on uncoiling and a decrease on 

 coiling take place with lead, copper, German silver, alumi- 

 nium, and magnesium ; and also with the first few operations 

 on soft iron wire, after which the signs are completely changed. 



An increase almost invariably follows coiling and uncoiling 

 with zinc, but the effect of coiling varies from one half to one 

 thirtieth of the effect of uncoiling. 



The difference between coiling and uncoiling, or, we might 

 say, the amplitude of the differences of resistances, is greatest 

 in the case of magnesium and least in the case of aluminium ; 

 the latter, on referring to the curves, will be seen to alter very 

 little indeed. 



The amplitude of the differences in the case of hard- drawn 

 German silver is not so great after a few series ; and when 

 annealed, after showing great increases after standing at rest, 

 the final series approach very near to that of the hard-drawn 

 wire, which is strong evidence indeed that the wire has become 

 harder through repeated coiling and uncoiling. 



The length of the wires were very carefully taken before and 

 after the experiments, and the necessary calculations made for 

 alterations in length and cross-section according to the formula 



R oc -, in which R = the resistance, / = the length, and a = 



the area of the cross section. 



