408 



Mr. R. Shida. 



reckoned positive, and those indicating the attraction negative. The 

 headings 1, 2, 3, &c, nnder " Deflections," are not only to show the 

 order in which the experiments were performed, but to distinguish the 

 results for one magnetising force from those for another ; the exact 

 value of the magnetising force in each case will be shown presently. 



The first wire tried was a very soft iron (pure) wire,* supplied by 

 Johnson and Nephew, Manchester, and is named in the table "Dark 

 Wire," from its appearance. It was of No. 10 B.W.G., its breaking 

 stress being about 15 kilogs. The piece experimented on was a metre 

 long ; its radius, when carefully calculated from its weight and specific 

 gravity, was '0374 of a centimetre, and therefore its sectional area 

 was '00439 square centim. The weight which was used for the 

 operation of " Ons and Offs " was 8 kilogs., only with this exception, 

 that at the beginning, while the force magnetising the wire was that 

 due to the vertical component of the earth's magnetism alone, a weight 

 of 10 kilogs. was put on once or twice. The wire underwent an 

 elongation of 2 '9 per cent, of its original length, so that it was now 

 102 - 9 centims., and its sectional area '00425 square centim. ; the 

 elongation was permanent and constant, that is, the subsequent appli- 

 cation of 8 kilogs. produced no more effect as to elongation. The 

 results for this wire are shown in the Table L In this table, the 

 results under the heading numbered 1, which are those for the 

 Glasgow vertical force, it must be mentioned, were obtained after the 

 wire had been treated in the following manner : — The operation of 

 " Ons and Offs," of a weight of 8 kilogs., having been performed 

 while the wire was hanging one way, say, with the end A up, its 

 magnetisation was observed in the manner explained before; the 

 wire was then inverted, and the operation of " Ons and Offs " was 

 again performed while it was hanging with the end A' up, that is 

 while the vertical force was acting in the opposite direction with 

 respect to the wire, and its magnetisation was again observed ; this 

 process was repeated until the magnetisation of the wire in the two 

 cases was equal, or nearly so, in intensity, but opposite in polarity. 

 The first and second columns under any of the headings numbered 

 1 to 8 give the result obtained in the two cases respectively : (1) 

 while a weight of 8 kilogs. was actually hanging on the wire (a case 

 to be hereafter denoted by " On "), and (2) while the weight was off (a 

 case to be hereafter denoted by " Off") ; and the third column, if any, 

 contains the result obtained for the effect of the coil alone carrying a 

 current. The first column under 12 and 13 contains the result ob- 

 tained (in the case "Off") immediately after reversing the current 

 in the coil, the operation of " Ons and Offs " having been of course 

 performed before the current was reversed ; while the second and 



* This wire is of the same kind as that used in the experiments described in Sir 

 William Thomson's paper, " On the Electrodjnamic Qualities of Metals, Part VII." 



