﻿486 Mr. M. H. Belz on the Heterodyne Beat Method 



determine the effect of this shield on the strength of the 

 magnetic field within. The magnetic force, H t , at a depth t 

 in a mass of metal is related to the force, H , at the surface 

 by the equation 



H, = IV 



V -ST-' 



in which //,, cr represent the permeability and specific 

 resistance respectively of the metal, and p = 27rn, n being 

 the frequency. Taking w = 4'84 x 10° per second, the largest 

 frequency used, and for platinum, /jl=1, cr = ll,000 c.g.s. 

 e.m.u., we find that within the shield for t = 7 x 10~ 6 cm., 



H,/H = 6- 0000292 , 



= 0-9997. 



The effect of the field on the changes of inductance can thus 

 be neglected. 



With the shield, the remaining causes of the change of 

 frequency are due entirely to changes of inductance, being 



(b) an eddy current effect within the specimen, and 



(c) in the case of the magnetic substances, a susceptibility 

 effect. 



The Eddy Current Effect. 



The magnetic field, H, within the coil will be of a 

 harmonic type, and on this account circular eddy currents 

 will be induced in the specimen in planes perpendicular to 

 the axis of the coil- and in such a direction that the magnetic 

 forces arising from them oppose and consequently diminish 

 the value of H. This virtually means a diminution in the 

 inductance of the coil, and the frequency change will be in 

 the direction of n increasing. This result can also be 

 predicted mathematically by regarding the specimen as 

 equivalent to a coil with self-inductance and resistance, 

 coupled to the main coil. The analysis, however, is com- 

 plicated and its development was not proceeded with. Some 

 experiments were made, however, to determine the order of 

 the change in inductance and its dependence on the charac- 

 teristics of the specimen. 



In the first place, as bearing on the results obtained in 

 the magnetic measurements, sulphuric acid vas examined. 

 This was contained in a long glass tube, the effect of which 

 had previously been determined to be zero, and was lowered 

 into the oscillating coil L 2 . Any change in inductance due 

 to an eddy current effect should certainly depend on the 



