Rayleigh's Address to the British Association. 95 



dynamo machine, the iron of the field-magnets approaches a state 

 of saturation, and the very elements of an explanation of the 

 action require us to take the fact into account. It is indeed 

 probable that a better knowledge of this subject might lead to 

 improvements in the design of these machines. 



Notwithstanding the important work of Rowland and Stoletow, 

 the whole theory of the behaviour of soft iron under varying 

 magnetic conditions is still somewhat obscure. Much may be 

 hoped from the induction- balance of Hughes, by which the mar- 

 vellous powers of the telephone are applied to the discrimination 

 of the properties of metals as regards magnetism and electric- 

 conductivity. 



The introduction of powerful alternate currents, in machines by 

 Siemens, Gordon, Ferranti, and others, is likely also to have a 

 salutary effect in educating those so-called practical electricians 

 whose ideas do not easily rise above ohms and volts. It has long 

 been known that when the changes are sufficiently rapid, the 

 phenomena are governed much more by induction, or electric 

 inertia, than by mere resistance. On this principle much may 

 be explained that would otherwise seem paradoxical. To take a 

 comparatively simple case, conceive an electro-magnet wound with 

 two contiguous wires, upon which acts a given rapidly periodic 

 electro-motive force. If one wire only be used, a certain amount 

 of heat is developed in the circuit. Suppose now that the second 

 wire is brought into operation in parallel — a proceeding equiva- 

 lent to doubling the section of the original wire. An electrician 

 accustomed only to constant currents would be sure to think that 

 the heating effect would be doubled by the change, as much heat 

 being developed in each wire separately as was at first in the 

 single wire. But such a conclusion would be entirely erroneous. 

 The total current, being governed practically by self-induction 

 of the circuit, would not be augmented by the accession of the 

 second wire, and the total heating effect, so far from being doubled, 

 would, in virtue of the superior conductivity, be halved. 



During the last few years much interest has been felt in the 

 reduction to an absolute standard of measurements of electro- 

 motive force, current, resistance, etc., and to this end many 

 laborious investigations have been undertaken. The subject is 

 one that has engaged a good deal of my own attention, and I 



