460 



Prof. D. E. Hughes. 



[May 27, 



protection, and therefore such lines must feel all the deleterious 

 effects that I have shown to be caused by the reaction of circular 

 magnetism. 



Influence of Copper and Iron Cores upon the Induction and 

 Resistance of Coils. 



It is well known that a coil of wire has a higher self-induction 

 than the same wire in a single loop, and that the coil has a still higher 

 induction when we introduce an iron core. I have made, however, a 

 series of experiments in order to measure the influence of a core upon 

 the resistance in the variable period, and also note the influence 

 exerted by the induced or eddy currents circulating in the core : the 

 following table shows some comparative results. 



Table IV. 



Helix formed of 









Per- 





insulated copper 



Comparative 



Resistance 



Resistance 



centage of 



Absolute 



wire, r50 metre 



force of the 



in ohms, 



in ohms, 



increased 



increase of 



in length, 2 cm. 



extra 



stable 



variable 



resistance 



resistance 



diameter, and 



currents. 



period. 



period. 



in variable 



in ohms. 



24 turns. 





period. 







460 



02632 



-02896 



10 



-00264 



Same with a core 













of solid copper. . 



352 



0-02632 



-04013 



52 



0-01381 



Same with a core 













of insulated cop- 















460 



-02632 



-02896 



10 



-00264 



Same with a core 













of solid soft iron 



2338 



02632 



-09870 



275 



07238 



Same with a core 













of 445 separate 













fine iron wires, 













each 0*25 mm. 















5360 



-02632 



-04448 



69 



0-01816 



Same with a core 













of silk-covered 













fine insulated 















5820 



-02632 



0-04075 



55 



-01443 



A helix was formed of an insulated silk- covered copper wire of 

 24 single layers, having an interior diameter of 2 cm., the object 

 being to form a coil having as little mutual induction as possible, but 

 which would be readily acted upon by any core of metal introduced ; 

 and in order to measure the high forces obtained an induction balance 

 of great power and range was used, the method, however, being the 

 same as that already described. 



The helix alone showed an induction value for its extra currents of 



