and Resistance of Straight Conductors. 389 



the case of an iron wire of *4 centim. diameter. The specific 

 resistance of iron in C.Gr.S. measure is about 10*; so that 



R 10 4 



I 7T X -04 



Thus, if p = 2tt/t, t being the complete period, 

 1 p 2 V_ 5-2 tf 



12 R 2 10 10 r 2 ' 



(22) 



The value of (m is more difficult to assign. It must be 

 remembered that it is only in a very rough sense that //, can 

 be treated as a constant at all. For small degrees of mag- 

 netization and for solid iron we may perhaps take //, = 300. 

 In the case of the hissing sounds of a microphone-clock, work- 

 ing by a scraping contact, r must be less than jooo second. 

 Taking it, however, at this value, we get 



1 p 2 Pa 2 

 J2 R^= 47 near1 ^ 



which shows that under these circumstances the resistance 

 and self-induction are entirely different from what they would 

 be for slow rates of alternation. 



We will now consider the case of copper, where p=l. 

 The specific resistance may be taken to be 1640. If a be the 

 radius in centimetres, 



1 p 2 l 2 fj? 7r 4 a 4 



1-2 a 4 



12 R 2 3 1640V 



iov " * • 



(23) 



From an alternate-current machine we may have currents 

 of period '01 second (100 positive and 100 negative pulses 

 per second). In such a case our fraction becomes *12 a 4 . 

 This shows that for diameters of 1 centim. and over, the 

 augmentation of resistance in the mains of an alternate- 

 current system will rapidly become of commercial importance. 

 A remedy may be found in the use of a more elongated 

 section, or in subdivision of the main into a number of detached 

 parts. 



In physical experiments, such as those in which absolute 

 resistance is determined by the method of the revolving coil *, 

 (23) may be neglected, as we may see by supposing a=^ } 



The ultimate form of (15), when p is very great, may be 



* See for example Phil. Trans. 1882. Experiments to Determine the 

 Value of the B.A. Unit of Resistance in Absolute Measure. 



