230 Lord Hayleigh on the Behaviour of Iron and Steel 



until it measured 26 divisions instead of 5 only. But a 

 similar series of operations with reversed currents (— ) caused 

 no swing amounting to y 1 ^ division ; so that we may consider 

 the compensation proved to be still perfect to about 1 per cent. 



In applying the method to still smaller forces we cannot 

 avoid a loss of sensitiveness. With a =100, ( + ) gave 3 di- 

 visions, while the effect of ( — ) remained insensible. The 

 correctness of the compensation is thus verified to about 6 per 

 cent, of the separate effects. Had the iron, even at this stage, 

 refused to accept magnetization, the fact would have mani- 

 fested itself by the equality of the swings obtainable in the 

 two ways, ( + ) and (— ), of making the connections. 



In the last case mentioned the current was 10~ 7 C.G.S., 

 and the magnetic force was 4 x 10~ 5 C.G.S. We may there- 

 fore regard the proportionality of magnetic induction to mag- 

 netic force over the range from \ H to ^pHasaii experi- 

 mental fact. In view of this, neither theory nor observation 

 give us any reason for thinking that the proportionality 

 would fail for still smaller forces. 



Quite similar results have been obtained with steel. On 

 December 13 a piece of drill steel (unannealed) was examined, 

 the delicacy of the apparatus, as evidenced by the ( + ) effect, 

 being about the same as in the above experiments on hard 

 Swedish iron. No failure of proportionality could be detected 

 with forces ranging from about £ H to xo o o o E. 



Annealed iron is a much less satisfactory subject. With 

 unannealed iron and steel the compensation for small forces 

 may be made absolute, so that neither at the moment of closing 

 the circuit nor afterwards is there any perceptible disturbance. 

 This means that (so far as the magnetometer-needle can 

 decide) the metal assumes instantaneously a definite magnetic 

 condition which does not afterwards change. But soft iron 

 shows much more complicated effects. The following obser- 

 vations were made upon a piece of Swedish iron (from the 

 same hank as the former) annealed in the flame of a spirit- 

 lamp. When an attempt was made to compensate for the 

 imposition of a force equal to \ H, no complete balance could 

 be obtained. When the coil was so placed as to reduce as 

 much as possible the instantaneous effect, there ensued a drift 

 of the magnetometer-needle represented by about 170 divi- 

 sions of the scale, and in such a direction as to indicate a 

 continued increase of magnetization. Precisely opposite 

 effects followed the withdrawal of the magnetizing force. 

 The settling down of the iron into a new magnetic state is 

 thus shown to be far from instantaneous. On account of the 

 complication entailed by the free swings of the needle, good 



