38-4 



PROFESSOR KNOTT ON SOME RELATIONS BETWEEN 



Before Strain. 



After Strain. 



T 



lO*-0 



T 



1O 4 



160 



657 



1155 



1652 



2150 



543 

 3-68 

 1-79 

 1-88 ? 

 103 



206 



847 



1487 



2121 



4150 



059 

 •40 

 •43 

 •43 

 •19 



These twists are produced by the combination of a longitudinal field of 2 7 "4 electro- 

 magnetic (C.G.S.) units, with a current along the wire of 1*52 amperes. The radius of 

 the wire, originally '0253 cm., was after the strain permanently reduced to '0223 cm. 



It is at once apparent that the stretched wire twists much less than the unstretched 

 wire. The marked diminution in the radius is, of course, a sufficient indication of the 

 great molecular change which stretching has produced in the wire. The wire has no 

 doubt been considerably hardened by the process, and is no longer to be regarded as the 

 same material. It should be mentioned that the numbers just given are only samples of 

 the observations taken. A comparison of the twists in other fields than the one chosen 

 leads, however, to the same general conclusion. 



It remains, finally, to consider the influence of change of temperature on the Wiede- 

 mann effect. In order to carry out such an experiment, it was necessary to coil the 

 magnetising helix upon a double-walled tube, between the concentric walls of which 

 steam or other vapour could be passed. The upper end of the bore through which the 

 wire hung was plugged up with cotton wool, so that no current of air could pass up or 

 down. Under these conditions, the temperature of the wire may be assumed to be not 

 very different from the temperature of the vapour after that has been for some time passed 

 through the space within the walls. In the experiments as conducted, steam at 100° C. 

 and water at 11° C. were passed in succession through the double- walled tube; and 

 observations on the twist made in the usual way. The following table (Table III.) gives 

 a few specimen numbers for both iron and nickel. 



As before, C is the current along the wire, and H 7 the corresponding magnetic force 

 at the circumference of the wire. H is the longitudinal field. H and IT are measured 

 in electromagnetic (C.G.S.) units. 



A glance at these numbers shows that the general tendency is for the twist to 

 diminish as the temperature is raised, although at the highest tensions for iron there 

 seems to be a tendency the other way. There is nothing as yet known regarding the 

 influence of temperature on the simple Joule effect ; but we might safely argue from the 

 results just given that in general change of length of iron and nickel in a given longi- 

 tudinal field is greater at the lower temperature. 



In discussing these experiments, I have throughout assumed the truth of Maxwell's 

 explanation of the Wiedemann effect in terms of the Joule effect. Wiedemann himself, 



