1886.] On the Self-induction of an Electric Current. 463 



768 double vibrations, the second C sharp or 576 double vibrations, 

 and the third, one octave below the first or Gr 384 double vibrations 

 per second. 



It will be seen that a great change in the variable resistance takes 

 place in the range of a simple octave, and that the fall from the wires 

 of 6 mm. to that of 1 mm. is so regular as to be almost a continuous 

 curve, and the total curve would be probably that taken by the wire 

 of 6 mm. in its approach to its stable resistance. If this is the case, 

 as future researches may show, then the resistance of this wire with 

 the comparatively slow contact speed of six per second would still 

 have 14 per cent, higher resistance in the variable than in the stable 

 period, or an ordinary Morse telegraph instrument working at a slow 

 speed of only eleven words per minute would experience this addi- 

 tional resistance ; consequently this effect should be easy to prove by 

 the ordinary methods with galvanometers, and this can be accom- 

 plished as soon as we have found a method of completely separating 

 the extra current from that due to the increased resistance, as I have 

 already succeeded in doing by the method which I have described. 



The iron wire of 6 mm. diameter shows for a speed of 384 contacts 

 per second a comparative resistance in the variable period of 638, or 

 more than six times its stable resistance, but with 192 contacts per 

 second its resistance is but 371 ; the fall of resistance is so rapid here 

 that for a single octave difference in the telephones, the fall is far 

 greater than the whole stable resistance. The extra current, as well 

 known, is proportional to the length of contact for fine wires, but in 

 large wires the curve indicates that the extra currents have a local 

 reaction on the cessation of the primary current. 



Influence of a Magnetic Sheath. 



The remarkable phenomenon wherein a non-magnetic metal such as 

 copper shows a higher " variable " resistance when under the influence 

 of an external coating of iron than a wire of solid iron (and this not- 

 withstanding that the resistance of the side of the bridge is kept a 

 constant or by balancing its resistance on the opposite side GD as 

 mentioned) seemed worthy of experimental research. 



Experiments were made in order to find what form of conductor 

 would give the maximum increased resistance in the variable period, 

 and I found that an external tube of iron insulated from its central 

 core produced this effect, for if the tube was not insulated but joined 

 in electrical contact the results were reduced, due I believe to a 

 transversal neutralisation taking place in its own portion of the wire, 

 as I have mentioned in my reply to the discussion on my first 

 paper. 



Table III shows the effect produced upon an electric conductor by 

 surrounding it with an insulated sheath of iron. It will be seen that 



