Determinations of Magnetic Susceptibility. 



411 



nexions of the electrodes of the coil, the galvanometer, &c, being 

 precisely the same as before. 



The same procedure in experimenting as before was followed as far 

 as possible ; that is to say, the bar and the coil, having been placed high 

 up to begin with, were lowered 2 centims. by 2 centims. until they 

 were low down, while the deflections of the magnetometer needle were 

 read for all the positions of the bar or the coil. In the case, however, 

 where this procedure was hardly possible, or, at any rate, hardly 

 worth going through, on account of the rapid variation of the current 

 in the coil, arising partly from the heating up of the coil and partly from 

 the polarisation of the battery (which coDsisted either of the Thomson 

 tray, Daniell's, or of the Faure accumulators, the latter being chiefly 

 used to obtain very high magnetising forces), the experiment was 

 made in the following manner : — A point of the bar, 28 centims. dis- 

 tant from its centre, having been placed on a level with the magneto- 

 meter needle (as this position of the bar was such as to give the 

 needle a maximum deflection for a high magnetising force), a strong 

 current was allowed to pass through the coil, and as soon as the 

 deflection of the needle was readable with a tolerable accuracy it was 

 read off at a certain moment by one observer, while the strength of 

 the current was measured by taking the reading of the galvanometer 

 at the same moment by another observer on word of command from 

 the former; the data thus obtained will, as we shall see, afford 

 the means of determining approximately the magnetisation of the 

 bar. 



The results of experiments on the bar of cast iron, steel, and malle- 

 able iron, are given in the Tables V, VI, VII respectively, the general 

 explanation of which has been already given in dealing with the other 

 tables. The first column under any of the headings 1, 2, 3, &c, in 

 each of the Tables V, VI, VII, contains the results obtained while 

 the magnetising force was in action ; while the second column, if 

 there be one, contains the result obtained directly after the with- 

 drawal of the force. 



Now the best way to study the results given in all the Tables I 

 to VII, is to plot curves in such a manner that the ordinates represent 

 the " Distances " of the centre of the wire or bar from the datum line, 

 the level of the magnetometer needle, and the abscissas represent the 

 " Deflections " of the needle in the scale divisions. To illustrate this, 

 the results shown in the second and third columns under 7, Table I, 

 are exhibited by the curves 1 and 2 respectively, Plate 10, in which 

 those distances measured upward from the datum line are reckoned 

 positive and those measured downwards negative ; while those deflec- 

 tions indicating the repulsion of the red end of the needle are reckoned 

 positive, and those indicating the attraction negative, according to 

 the convention already adopted. 



