184 C. G. KNOTT 



if the numbers for ± 120° and ± 180° are compared. Thus, some- 

 where between these two twists just named, there is a critical twist for 

 which the wire would probably show no magnetic lag at all. It will 

 be noticed how the curves open out as the twists are taken higher and 

 higher. They open out much more rapidly than they appear to do in the 

 Plate ; since, for convenience of representation, the curve for ±360° 

 is plotted to half-scale as regards twist, and the curve for ± 5 7T (shown 

 dotted in the diagram) to a quarter scale. Here also, as in the curves 

 for the circularly magnetised wire, the range of variation of the mag- 

 netic intensity increases rapidly with the twist for small twists, but 

 comes practically to a limit just about the critical twist for which the 

 magnetic lag vanishes. In a series of similar experiments on per- 

 manently magnetised nickel wire, Mr. Iwagawa failed to obtain any 

 indication of change of sign. The permanent magnetism rapidly 

 diminished as the twistings were taken larger and larger ; the range 

 during twisting reached a distinct maximum for a twist of ± 90°, and 

 then rapidly fell off, so that for a twist of ± 360° it was hardly mea- 

 surable. But throughout, the magnetic lag was always positive. 



I now pass to the results for iron. Some of these are given in 

 Table IV. arranged exactly as were the numbers for nickel in Table I. 

 It hardly seems necessary to give the curves. Enough to say that 

 they are very smooth, very similar in general outline to the corres- 

 ponding curves for nickel, but differ from these in being so to speak 

 their inversion, such as would result from reflection in a plane mirror. 

 The numbers in the fourth column are obtained by subtracting the 

 second column from the first, so that for true lag the differences are 

 positive. The wire used was 58 centimetres long and 0*64 millimetres 

 in diameter. The current along the wire was 1*4 ampères. The in- 

 tensities are given in approximately absolute electromagnetic units 

 (C. G. S.) 



