490 



Lord Rayleigh on the Self-induction and 



due to its magnetic quality, are doubtless disadvantages from 

 a telephonic point of view. If found serious they may be 

 mitigated, as Prof. Hughes has shown, by the use of a 

 stranded wire, in which the circumferential magnetic circuits 

 are interrupted. There has been some confusion, I think, in 

 connection with the notion of u retardation." If we had 

 the means of observing the passage of signals at various points 

 of a long cable, we should find them not merely retarded 

 (which would be of no consequence) as we recede from the 

 sending end, but also attenuated. The amplitude of a periodic, 

 e. g. telephonic, current sent into a cable becomes less and 

 less as the distance increases. Nothing of the kind can 

 happen in a well-insulated iron wire of negligible electrostatic 

 capacity. Its resistance and self-induction may oppose the 

 entrance of a current, but whatever current there is at any 

 moment at the sending end of the wire must exist unimpaired 

 throughout its whole extent. 



I will now record a few experiments as to the effect of an 

 iron core upon the apparent self-induction and resistance of 

 an encompassing helix. The wire was wound in one layer 

 upon a glass tube ; the total number of turns is 205, occu- 

 pying a length of 28 # 6 centim. The length of the wires 

 forming the cores was 24*1 centim. The results given are 

 the differences of the readings obtained with and without the 

 cores, so that the resistance and self-induction of the helix 

 itself are not included. The interrupter was the same reed as 

 in previous experiments. 



A comparison was made of the effect of a solid iron wire 

 1*2 millim. in diameter and of two bundles of wires of similar 

 iron (drawn from the same specimen) of equal aggregate 

 section and weight. One bundle contained 7 wires, and 

 another 17. The results were : — 





1 wire. 



7 wires. 



17 wires. 



Resistance 



1-3 

 13° 



0-3 



18° 



0-2 



18° 



Self-induction 





showing that when the wire was undivided the secondary 

 currents developed in it increased the apparent resistance of 

 the helix by 1*3, and 'diminished the apparent self-induction. 

 A similar experiment was tried with a stouter wire, 

 3*3 millim. in diameter (from the same hank as the length of 

 18-34 metres treated as a conductor). In the hard condition 



