226 Lord Rayleigb on the Behaviour of Iron and Steel 



in the coils of an induction-balance. Experiment showed 

 that iron responded powerfully to somewhat feeble forces ; 

 and I endeavoured to improve the apparatus in the hope of 

 being able thus to examine the subject more thoroughly. 

 Two similar long helices were prepared by winding fine insu- 

 lated wire upon slender glass tubes. These were connected in 

 series with a battery, a resistance-box, and a microphone- 

 clock, so as to constitute a primary circuit. The secondary 

 consisted of a large quantity of copper wire, mounted upon a 

 bobbin, through the opening in which both primary coils were 

 inserted. The circuit of the secondary was completed by a 

 telephone. When neither primary coil contained a core, 

 silence at the telephone could readily be obtained. The iron 

 cores used were those described in Part II."*; and it was found 

 that all of them (including the bundle of seventeen very fine 

 wires) disturbed the silence until the resistance was so far 

 increased that the magnetizing force was less than about ^ of 

 the earth's horizontal force (H). Moreover, there was no 

 indication that the absence of audible effect under still smaller 

 magnetizing forces was due to any other cause than the want 

 of sensitiveness of the apparatus. 



I did not pursue the experiments further upon these lines, 

 because calculation showed that the feeble magnetization of a 

 piece of iron could more easily be rendered evident directly 

 upon a suspended needle (the magnetometric method), than 

 indirectly by the induction of currents in an encompassing coil 

 connected with a galvanometer. Nearly all the results to be 

 given in this paper were obtained by a form of the magneto- 

 metric method, specially adapted to the inquiry whether or not 

 the magnetization of iron continues proportional to the mag- 

 netizing force when the latter is reduced to the uttermost. 



The magnetizing-spiral first used was one of those already 

 referred to. It consists of a single layer of fine silk-covered 

 copper wire wound on a glass tube and secured with shellac 

 varnish (A, Plate IT. fig. 1). The total length of the spiral 

 is 17 centim., its diameter is about *6 centim., and the wind- 

 ings are at the rate of 32 per centim. The resistance is about 

 5J ohms. 



The magnetometer was simply a small mirror backed by 

 steel magnets (B), and suspended from a silk fibre, as supplied 

 by White for galvanometers. It was mounted between glass 

 plates at about 2 centim. distance from the magnetizing-spiral. 

 The earth's force was compensated by steel magnets, which 

 also served to bring the mirror perpendicular to the helix in 

 spite of the influence of residual magnetism in the iron core. 

 * Phil. Mag. December 1886, p. 490. 



