340 



SCIENCE 



[N. S. Vol. LIV. No. 1398 



the substance, placed in a magnetic field of 

 strength, E, then we have the relation exist- 

 ing between these quantities given by the 

 equation 



B=H±4:ttI. (1) 



The number of lines of force which thread 

 out from a magnetic pole is 4irm. In equation 

 (1) B is less or greater than H" as Z is nega- 

 tive or positive. That is to say, there is 

 developed at opposite ends of the specimen 

 placed in the magnetic field, H, a polarity 

 which in case of paramagnetic substances is 

 additive to E and makes B greater than E 

 while in diamagnetic substances an opposite 

 polarity is developed whose field subtracts 

 from E and makes the resultant lines of 

 induction further apart than the lines in the 

 field of force. I, therefore, may be defined 

 as the pole strength per unit area of the pole 

 developed in the specimen, or it is the in- 

 tensity of magnetization of the material ex- 

 amined. More frequently I is defined as the 

 magnetic moment per unit volume, for if we 

 take a cylinder of any material and place it 

 in a magnetic field, then AIl^M, the 

 magnetic moment of the cylinder, where A is 

 the crossection and I is the length of the 

 cylinder. I=-M/Al^M/y, or the magnetic 

 moment per unit volume. It is assumed that 

 the poles are at the ends of the cylinder. 

 Next divide equation (1) by E and we get 



ix = l±i-rr-k; (2) 



where /* is called the permeability, and h the 

 susceptibility. jx^B/E is a measure of the 

 power the substance has for increasing the 

 external field. This is a quantity in which 

 the electrical engineer is particularly inter- 

 ested. Further, h^I/H seems also to be. a 

 factor due to properties inherently bound up 

 with the substance introduced into the magne- 

 tic field. This factor fc is called the magnetic 

 susceptibility per unit volume. In order to 

 get the susceptibility per unit mass we must 

 divide the volume susceptibility by the 

 density of the substance. As k is negative or 

 positive so is a substance dia- or paramag- 

 netic. It is a property in which physicists 

 must be vitally concerned in building up a 

 magnetic theory and developing comprehen- 



sively the architectural design of the atom. 

 Before we have finished this discussion we 

 must ask the question, where does the property 

 of susceptibility lie — in the electron, atom, 

 molecule or aggregation of molecules? 



2. Next let us work with a non-uniform 

 magnetic field such as one has between the 

 conical pole-pieces of an electromagnet and 

 let us give definite shape to the samples of the 

 various materials investigated, viz., ellipsoidal 

 form. This time we will observe the behavior 

 of the specimens as the magnetic field is ap- 

 plied to them. Experimentally, we discover 

 that here again all substances divide them- 

 selves into two groups; one class turns in the 

 magnetic field so as to set the greatest length 

 normal to the lines of force of the magnetic 

 field and the other class with major dimen- 

 sions parallel to the field. Not only that but 

 those substances which set themselves normal 

 to the field are just those which we call dia- 

 magnetic in our first experiment and those 

 which turn with greatest length parallel are 

 the paramagnetic elements, which also include 

 the ferromagnetic substances. Thus we have 

 another way in which to distinguish dia- from 

 paramagnetic substances. It is to be noted 

 that in a uniform magnetic field all elongated 

 bodies set themselves parallel to a magnetic 

 field. The reason for the orientation cited 

 above for diamagnetism is because the poles 

 of the substance tend to move from stronger 

 to weaker fields. ^°- 



3. As a third experiment let us work with 

 a non-uniform magnetic field in which the 

 variation of the field along any direction is 

 known. Introducing our samples in the form 

 of spheres into this field we note that they all 

 tend to move in one direction or the other in 

 the field, either from a point of large field 

 intensity to one of lower or vice versa. As in 

 our previous observations there are two classes 

 and we find that diamagnetic substances 

 always move from higher to lower field in- 

 tensities and paramagnetic are urged in the 

 opposite direction. Ferromagnetic bodies 



la Poynting and Thomson, Elec. and Maa., p. 

 258, 1914. 



