488 Lord liayleigh on the Self-induction and 



in the above examples, is an extreme case of what may happen 

 to a sensible extent within a simple conducting cylinder, 

 especially of iron, when the diameter is not very small in 

 relation to the frequency of electrical vibration. In order to 

 avoid magnetizing the material of the conductor, the current 

 tends to confine itself to the outer strata, in violation of the 

 condition of minimum resistance. Prof. Hughes has already 

 given examples of this effect ; but they are difficult to com- 

 pare with theory in consequence of his employment of a 

 vibration of indefinite pitch. The following observations were 

 made with the usual reed interrupter, giving about 1050 

 vibrations per second. 



A somewhat hard Swedish-iron wire, 10*03 metres long, 

 and 1*6 millim. diameter, was first examined. The resistance 

 to steady currents was 10*3, and to the variable currents given 

 by the reed, 12*0. The wire was then softened in the flame of a 

 spirit-lamp, after which the resistance to steady currents was 

 10*4, and to variable currents 12*1. Expressed in ohms, the 

 resistance to steady currents is 



10-4 x -0492 = -51 ohm. 



From these data we may deduce an approximate value of 

 the magnetic permeability (jjl) of the material for circumfer- 

 ential magnetization. For if I be the length, R the resistance 

 to steady currents, p/27r the frequency of vibration, we have 

 for the resistance (E/) to variable currents the approximate 

 expression * 



K-K|l+ 12 K2 lg0 Ri +...}, 



so that for the rough determination of fju we may take in the 

 present example, 



1 pHV _ 1-7 

 12 R* 10-4' 



The result is ^ = 108. 



A more accurate use of the formula would bring out a 

 sensibly higher value ; but it is hardly worth while to pursue 

 the matter, inasmuch as an}^ deduction of fju from the small 

 observed difference of resistance (1*7) is necessarily subject to 

 considerable error. 



In order to get better materials for a determination of //, by 

 this method, a stouter wire of Swedish iron was next tested, 

 18*34 metres in length and 3*3 millim. in diameter. The 

 metal was rather hard. The resistance to steady currents 

 was found to be 4*7, and to the variable currents from the 

 * Phil. Mag. May 1886, equation (19) p. 387. 



