Determinations of Magnetic Susceptibility. 



437 



deflections due to the coil being taken into consideration), corre- 

 sponding to the distance 28 centims. (a distance approximately 

 corresponding to a maximum deflection for high magnetising forces) 

 are proportional to the intensity I. The deflections due to the coil 

 alone were calculated from the strength of the current in the coil after 

 a manner to be discussed later on. 



Just a few words are perhaps necessary to explain the details of the 

 Tables A, B, C, &c. In the first place the results given in the first, 

 second, third, &c, horizontal lines along with the numbers 1, 2, 3, &c, in 

 the Tables A, B, C, &c, correspond to the first, second, third, &c, vertical 

 columns under the headings 1, 2, 3, &c, in the corresponding Tables of 

 Deflection I, II, III, &c. No sign or a negative one is prefixed to the 

 numbers, according as the polarities of the wire or bar or coil were 

 similar or dissimilar to those induced in the wire by the earth's 

 vertical force alone, if the numbers refer to the quantities indicating 

 the magnetisation ; or according as the magnetising forces were in a 

 similar or dissimilar direction to the vertical force, if the numbers 

 refer to the quantities representing the magnetising forces either 

 directly or indirectly. Again, it will be observed that in 7 and 8 of 

 the Table A, and of the Table C, there were obtained two values for 



-5-, one calculated and the other observed ; the object of this was two- 

 fold : (1) To insure that the calculated value was within the errors of 

 observations in the measurements of the current strengths, the 

 dimensions of the coil, &c. ; and (2), to render the results for this 

 maximum magnetisation of the wires corresponding to these tables 

 independent of the accuracy or inaccuracy of the measurement of the 



current strengths ; the observed value for Q.m f was used, in these cases, 



to evaluate the quantities I, &c. 



The rest of what is given in the Tables A, B, 0, &c, will, I hope, 

 explain itself. But by far the readiest mode of studying the whole 

 results, is to refer to the graphical representation shown in the Plates 

 11, 12, 13, 14, 15, the first three and the fourth of which contain the 

 curves representing the "intensity of magnetisation" and the "mag- 

 netic susceptibility " respectively of the wires, and the last contains the 

 curves representing the " intensity of magnetisation " of the bars. In 

 other words, the curves in the Plates 11, 12, 13, and 15 are so drawn 

 that the abscissas are proportional to the magnetising force F, and the 

 ordinates to the intensity I ; whereas in the curves in the Plate 14 

 the abscissas and the ordinates are proportional to the force F and the 

 susceptibility /x respectively. 



As regards, first of all, the Plates 11, 12, 13. The curves in the Dia- 

 gram I correspond to the " Dark Wire," those in the Diagram II to the 

 " Bright Wire," and those in the Diagram III to the " Steel Pianoforte 



2 g 2 



