CHAPTER XXII 



MEASUREMENTS OF SUSCEPTIBILITY AND 



PERMEABILITY OF PARAMAGNETIC 



AND DIAMAGNETIC SUBSTANCES 



Faraday's experiments Rowland's exprriiii>-iit ExjH'riiiHMits 



v. Ettingshausen Curie's experiment* Curie's law Wills' experiment 



Townsend's experiment Pascal's experiments The electron theory 



The magneton. 



THE earliest measurements of the magnetic qualities of stihstances 

 other than ferromagnetic \\civ made, MUMI after I-'aradas's diMo 

 of (liaiuagnetism, by Pliicker,* by K. Hcctjuercl.t and I) iv.J 



Though excelled in accuracy by later work, HUM- earl Y experiment! 

 arc worthy of attention, since tin- methods UM d an >imple in principle 

 and easily understood. Tliev consisted in su^u-nding a body at a 

 gixen point in the fit-Id between the poles of a strong magnet \ 

 to, but not quite in, the a\is, and in measuring either fa * tioMm 

 balance or by a torsion balance tin- force acting on the body. That 



force in any direction .r is **~* 1 - per unit \ohnne, \\h- 



is the susceptibility of the bodv and ^ that of th- -nrnmnding 

 medium, and H is the intensity of flu- field (p. ^~>^). If bodies of 

 the same \olmnr an- u^d m Mio-e^ion ut the same point in tin- 

 same field, the forces are proportional to Ar t K I or to Mt~Mr 



\Ve shall select for descript ion hen- the t-xpt riinen' ulay. 



It is to be noted that he ga\e an account of them only in a lecture 

 at the Royal Institution, and the account \vas e\i<lently inteiulfd 

 as provisional. But the work \va^ apparently new-r eontinued. 

 Faraday's experiments, laradav u-ed a large horsi 

 permanent magnet to give the field. The plan is -hown in 1 

 The body, shaped into a cyli IK K r, \\a> hung at ///. UMially (V5 in. fnm 

 tne strongest part of the field c. To measure the force on it, it 

 was hung by a fine glass fibre 5 in. long from one end of a torsion 

 rod parallel to ae, but with the end exactly over ///. A counter- 

 poise was attached to the other arm of the torsion rod. The 



. Ann., Ixxiv. (1849), p. 321. 

 t Ann. d Chan, et de Phy*., xxxii. (18:>1), p. 68. 

 EJP[). Ecs., iii. p. 497. 



282 



