492 



PROFESSOR KNOTT ON SOME RELATIONS BETWEEN 



but if Joule's conclusion as to the contraction of iron at right angles to the direction of 

 magnetisation be correct, we should expect a current along a wire to have a shortening effect 

 upon it. This shortening effect, if it exists, will depend on the value of the magnetising 

 force due to the current, and will be different for each cylindrical layer. The resulting 

 strain will be far from simple. An originally plane section will probably become warped, 

 the end becoming concave outwards. An experimental attempt to find evidence of such 

 a change of curvature in the polished end of an iron rod failed, possibly because I had no 

 means of getting a strong enough current along the rod. A current of 100 amperes along 

 a rod half a centimetre in diameter might be expected to give a good measurable effect. 



6. Results j 'or Nickel. — Before discussing other interesting features of the phenomenon 

 in iron, we shall give the results for nickel, so as to be able to draw comparisons between 

 the two metals. Two nickel wires of diameters 05 mm. and 0"94 mm. respectively, and 

 of length 77 cm., were studied in exactly the same manner as the iron. The results are 

 given in Tables II. and III., and Plate II. contains the curves corresponding to the latter 

 table. Similar curves may also be drawn for Table II., but as they lead to identical 

 conclusions it does not seem necessary to give them. The units used are the same as in 

 the case of iron, but the ordinates representing the twists in nickel are drawn to half the 

 scale of the same quantities for iron. It is necessary to bear this in mind in comparing 

 the curves for the two metals. 



Table II. — For Nickel Wire, 0*5 mm. in diameter. 



Tension Kg.-Wt. 

 per Sq. Cm. 



Line Current 

 in Amperes. 



Longitudinal 



Field Electrom. 



Units, C.G.S. 



Twist ± H in 



Thousnndths of 



Radians. 



Twist ± C in 



Thousandths of 



Radians. 



Experiment 

 Number. 



T 



c 



H 



ht 



ct 



No. 



924 



0-94 



5-2 

 9-9 



22-3 



38-5 



79-8 



143-8 



172-2 



0-46 

 1-21 

 7-13 

 14-21 

 25-87 

 25-92 

 22-99 



0-25 

 092 

 2-72 

 3-87 

 5-60 

 7-27 

 7-69 



(1) 



1155 



1-02 



9-2 



22-6 



40-4 



88-1 



166-2 



210-2 



Ml 



8-84 

 18-08 

 27-84 

 25-08 

 21-80 



1-02 

 3 05 

 4-35 

 6-46 

 8-03 

 8-15 



(2) 



1155 



1-68 



5-5 

 10-3 



24-8 

 40-7 

 68-8 

 98-5 

 165-1 

 1998 



1-04 

 5-63 

 19-38 

 29-48 

 36 00 

 37-69 

 33-44 

 30-63 



1-60 

 5-46 

 908 

 11-29 

 13-33 

 14-38 

 15-60 

 15-10 



(3) 



