262 Mr. C. Gr. Lamb on the Distribution of 



silvered quartz fibre, which, in addition to its great simplicity, 

 has the merit of exhibiting the phenomena in a remarkably 

 evident manner. 



In conclusion, it may be noticed that the attraction of 

 dissimilar metals, besides being observable experimentally 

 with comparative ease, furnishes a new method for the deter- 

 mination of the electromotive force of contact. This method, 

 which can be performed fairly quickly in all cases, is also 

 capable of considerable precision when suitably arranged, 

 since it is a null method. 



XXVII. On the Distribution of Magnetic Induction in a Long 

 Iron Bar. By C. Q. Lamb, M.A., B.Sc* 



THE following investigation was undertaken to determine 

 the distribution of induction in a long cylindrical iron 

 rod when it was subjected to various magnetizing forces. It 

 was felt that the great variation of induction which must 

 necessarily occur in a cylindrical bar would vitiate to some 

 extent the ordinary assumptions made when employing rods 

 for mao-netic measurements. 



In order to get a sufficiently clear notion of the induction 

 distribution, and how it is produced, it is desirable to determine 

 the following data: — 



(P) A curve of magnetization of the bar as determined by 



means of a search-coil at its centre. 

 (Q) A series of curves at various fixed magnetizing forces 



showing the distribution of induction in each case. 

 (R) A magnetization-curve of the bar when made into a 

 ring, so that the induction is the same at every cross- 

 section 

 The bar used in the [present case was a circular bar of 

 Low Moor iron ; it was 0485 cm. in mean diameter, and 

 12o*4 cm. (48 inches) long, and before experiment was very 

 carefully annealed. The magnetizing-coil was wound on a 

 brass tube, somewhat longer than the specimen and of larger 

 diameter : the specimen was fixed centrally inside this by 

 ebonite rings near the centre and at one end. The search- 

 coil was made as small as possible, and was wound on an 

 ebonite bobbin attached to the end of another brass tube of 

 such a size as just to slide over the rod, while the search-coil 

 could just slide inside the tube covering the magnetizing-coil: 

 on the inner tube marks were made showing when the search- 



* Communicated by the Physical Society : read June 9, 1899. 



