156 / MrVH. A. Rowland on Magnetic Permeability, 



proximations. Wc might indeed solve with reference to Q ; but 

 in this case some values of X as obtained from experiment may 

 be accidentally greater than A, and so give an imaginary value 

 toQ. 



By plotting any Table in this way and measuring the distance 

 C, we have the maximum of magnetism. 



I have given in the same Plate the curve drawn from the ob- 

 servations on the nickel ring with Q on the same scale, but X 

 on a scale four times as large as the other. The curve of nickel 

 satisfies the equation 



X=2120sin 



. /Q + 5-35X-f2000\ 

 H 360~ J 



quite well, but not so exactly as in the case of iron. This ring, 

 when closely examined, was found to be slightly porous, which 

 must have changed the curve slightly, and perhaps made it 

 depart from the equation. 



In Table VIII. I have collected some of the values of the 

 constants in the formula when it is applied to the different rings 

 and bars, and have also given some columns showing the maxi- 

 mum of magnetism. When any blank occurs, it is caused by 

 the fact that for some reason or other the observations were not 

 sufficient to determine it. The values of a, H, D, and the value 

 of X, when Q = 0, can in most cases only be considered ap- 

 proximate; for as they all vary so much, I did not think it 

 necessary to calculate them exactly. For comparison, I have 

 plotted I)r. Stoletow's curve and deduced the results given in 

 the Table, of course reducing them to the same units as mine. 



It will be observed that the columns headed tc maximum of 

 magnetism " contain, besides the maximum magnetic field, two 

 columns giving the tension of the lines of force per square centi- 

 metre and square inch of section of the lines. These have been 

 deduced from the formula given by Maxwell* for the tension 



per square metre, which is — absolute units of force. 



- This becomes 



Q2 



24655000 00 kilo S rammes P er s( l uare centim. 



173&000 lbs ' perS( l UareinC ^ 



(12) 



from which the quantities in the Table were calculated. 



It is seen that the maximum of magnetism of ordinary bar 

 iron is about 175,000 times the unit field, or 177 lbs. on the 



* Treatise on Electricity and Magnetism, vol. ii. p. 256. 



