Magnetic Induction in Straight Iron Rods. 



183 



From his experiments, Prof. Fleming deduced, that at a 

 point of the specimen whose distance from the centre is a 

 certain fraction of the half-length, the induction is always 

 equal to B,. 



The author has found this to be true for any one specimen 

 as long as the centre of the specimen is not saturated, but as 

 the centre becomes saturated, the distance from the centre 

 where the induction is equal to B 1 increases. 



Fig. 2 shows this very plainly. The position of this point 

 for different inductions is indicated by a cross. Table I. 

 gives the observations which are shown in fig. 2. 



Table I. 



Distance from 













centre of speci- 





Induction for different fields. 





men in centim. 

















1088 



6060 



10300 



12940 



14160 



3 



1070 



6025 



10180 



12860 



14050 



6 



1011 



5790 



9900 



12580 



13900 



9 



976 



5470 



9450 



12000 



13390 



12 



893 



4960 



8700 



11130 



12620 



15 



781 



4330 



7610 



9900 



11400 



18 



G55 



3510 



6090 



8090 



9530 



21 



420 



2470 



4230 



5760 



6890 



24 



204 



1070 



1906 



2490 



3040 



Table giving distribution of induction in specimen No. 1 for different fields. 



With regard to the relative position of this point for 

 different specimens, it was found that this was the same 

 for all specimens whose length is less than thirty times the 

 section, and that the relative distance from the centre in- 

 creased as the ratio of length to section increased. 



This is shown by Table II., which gives the results obtained 

 from specimens whose length is greater than thirty times the 

 section. 



Prof. Fleming found this distance to be '56 of the half- 

 length. The mean value deduced from Table III. for speci- 

 mens whose length is less than thirty times the section is*564. 



While the variations from this value, as shown by Table III., 

 are not more than 1 per cent., the error in the value of B l 

 may be much greater than this, if obtained by measuring the 

 induction at this point as suggested by Prof. Fleming ; for 

 since the rate of variation of the induction along the specimen 

 at this point is great, a small error in placing the coil would 

 give a relatively large error in the result. If. on the other 

 hand, the induction at the centre of the specimen is measured, 



