Magnetic Permeability of various Alloys of Iron. 



71 



different places by means of a micrometer-screw reading to one 0-01 millim., and 

 the mean of the readings was taken as the true diameter ; the diameters of tlie 

 standards were similarly determined once for all.* The conductivity of the test 

 rod was calculated in terms of pure copper, Matthiessen's standard of pure 

 copper being taken as 100; our own specimen was I'Ol per cent, higher con- 

 ductivity than Matthiessen'sf ; allowance for this was made in the following 

 calculation for the conductivity of the rods under test : — 



I = length of standard. 

 r = length of rod under test. 



d = deflection due to fall of potential in standard. 

 d' = deflection due to fall of potential in test rod. 

 a = cross-sectional area of standard. 

 a' = cross-sectional area of test rod. 

 c = conductivity of standard. 

 c' — conductivity of test rod. 

 , d I' a 



(^ = ITT-, C- 



a I a 



The following comparison of the iron standard with the copper standard will 

 serve as an example of the series of readings taken with all the rods : — 



Standaed Ibon (Temperature 15° C). 



from which the conductivity c' was deduced thus : — 



140-5 X 



X 0-0121 



101-01 



~ 54-8 X 100 X 0-1874 

 or conductivity of standard iron = 16-35, pure copper being taken as 100. 



* The cross-section of the rods was slightly elliptical, and, moreover, not quite uniform throughout 

 their length ; later on in the investigation, the mean cross-sectional area of each rod was more accurately 

 and expeditiously determined by measuring the length of the rod carefully, and then obtaining its volume 

 by displacement of water in a long glass tube, 1 cm. diameter, graduated to tenths of a c.c. This method 

 of measurement gave such excellent results that the conductivities of the whole of the annealed rods were 

 re-determined after this paper had been printed, and the corrected results were inserted in the proof, 

 wherever any sensible difference was found. Hence the unannealed results.are less reliable. 



f The standard copper was supplied by Messrs. White & Co., of Glasgow, and the conductivity of the 

 sample determined in Lord Kelvin's Laboratory at the University of Glasgow. 



