232 Mr. J. M. Baldwin on the Behaviour of 



decidedly less than this, it may be preferable to keep still ; 

 but we are then liable to serious errors, should the signal 

 happen to come from nearly in front or nearly behind. A 

 judgement that the signal is to the right or left may usually 

 be trusted, but a judgement that it comes from in front or 

 behind is emphatically to be distrusted. If, for example, the 

 sound seems to come from a position 45° in front of full right, 

 we must be prepared for the possibility that it is really 

 situated 45° behind full right. A combination of 3 or 4 

 observers facing different wa} T s offers advantages. A com- 

 parison of their judgements, attending only to what they 

 think as to right and left and disregarding impressions as 

 to front and back, should lead to a safe and fairly close estimate 

 of direction. 



Terlicg Place, Witham. 



-rr =- = ' $ 



XIII. The Behaviour of Iron under Small Periodic 

 Magnetizing Forces. Bi) J. M. Baldwin", M.A., B.Sc* 

 [Plate VII.] 

 1. rflHE object of the present paper, which was undertaken 

 A at the suggestion of Professor Lyle, is to investigate 

 the relations which obtain under actual working conditions 

 between the amplitudes and phases of the different harmonics 

 of the magnetic induction produced in iron, and those of the 

 magnetic intensity producing it, for low values of the induc- 

 tion ; and from the harmonic expressions obtained for these 

 quantities to deduce the loss of energy in the iron. The 

 work which has been done previously on this subject has 

 been performed under statical conditions, which are seldom 

 realized in practice. 



The induction produced in iron in weak magnetic fields 

 has been measured by Baur f using the ballistic method 

 (statical). For the range H = 038 to H = 0*016 (with 

 corresponding limits of the induction B = 283 and B = 3 - 3) 

 he found that the permeability fi could be represented closely 

 by an expression of the form 



/jb = a + &H, 



where a and /; are constants for the specimen. By extra- 

 polation, fju would tend to a definite limit in exceedingly weak 

 fields. This limit for the specimen examined was 185. 



Later, Lord Bayleigh J, working with the compensated 

 magnetometer method (statical), showed that, for unannealed 



* Communicated by the Physical Society : read October 26, 1906. 



f C. Baur, Wied. Ann. xi. p. 394 (1880). 



\ Lord Rayleigh, Phil. Mag. [5] vol. xxiii. p. 225 (1887). 



