MEASUREMENT OF PERMEABILITY 



269 



The following Table gives particulars of a test made by Ewing on 

 an annealed wrought-iron wire of diameter 0-077 cm. and length 

 30'5 cm. or 400 diameters. The current was increased step by 

 step from to a value giving a field of 22*27 gausses and was then 

 reduced by steps through to 2*87 gausses. The continuous 

 curves in Fig. 206 (taken from Ewing) show the values of I as 

 ordinates plotted against H' as abscissae, the lower curve for increas- 

 ing current and the upper one for decreasing current. 



The figure shows how the correction from H' to H may be 

 made graphically. Since the correction is 0-00045 I, draw a 

 >tr,-ii<rht line OE with abscissa 0'45 at 1 = 1000. The abscissa 

 of OE for any value of I will give the correction to H' at that 

 value. Then draw a new curve at each point a distance to the 

 left of the continuous one equal to the abscissa of OE at that 

 level, and this new curve will show the relation between I and H. 

 The dotted curves in the figure are thus obtained. The Tables 

 show that with a long thin wire the correction is not great. Th e 



