444 



SCIENCE. 



[Vol. VII., No lti 



copper and an iron wire of equal resistance, 1 

 metre in length, were measured for inductive 

 capacity and resistance, the capacity of the copper 

 wire being taken as 100, and the iron being 400. 

 The copper wire showed an increased resistance, 

 during the variable period, of 8 per cent, as com- 

 pared with 128 per cent for iron ; but a great 

 change took place when each of these was placed 

 in the interior of an iron gas tube of sufficient 

 diameter to allow of the wire being insulated. 

 The force of the extra currents in the copper wire 

 then increased 350 per cent, while in the iron 

 they increased 8 per cent, the force of the extra 

 currents being now, for copper 450, and for iron 

 433. 



The influence of an iron tube on the resistance 

 of the variable period was still more marked. The 

 copper wire which, without the exterior iron 

 tube, had only 8 per cent increase, now showed 

 934 per cent ; or, by direct measurement, 1 metre 

 of this wire, during the rapid rise and fall of the 

 current in the variable period, had a resistance 

 the same as 10.34 metres in the stable period, — a 

 much greater difference than was obtained with 

 iron wire, which only showed an increase of 22 

 per cent. Thus copper shows three times the sen- 

 sibility to an iron sheath which iron does, a fact of 

 importance in electrical engineering. Iron is 

 much less affected in self-induction by exterior 

 influence than copper. Copper coils are much 

 more sensitive to iron cores within them than 

 iron coils, and the resistance of a copper coil may 

 be in the variable period far more than that of an 

 equal iron coil, if an iron core react within it. It 

 is this fact, however, as Professor Hughes points 

 out, which enables copper coils to be so effective 

 in trans forming energy in ' secondary generators ; ' 

 and he remarks that a dynamo having its electro- 

 magnet and armature wound with insulated iron 

 wire, would, irrespective of its resistance, have 

 an extremely low efficiency as compared with one 

 wound with copper. As regards the resistance of 

 either of those wires, Professor Hughes observes 

 that there can be no doubt that the resistance of 

 the armature of a dynamo, or, in fact, of any coil 

 of wire, as measured during the stable period, 

 gives no approximate indication of what its real 

 resistance is during the period in which it is doing 

 work. This remark bears out a recent suggestion 

 to the effect that the resistances of conductors, 

 apparatus, and standards, as measured by battery 

 currents in the stable period, differ to some ex- 

 tent from their values when traversed by the 

 rapidly fluctuating currents of a dynamo. A 

 further investigation of the matter is required in 

 order to find out its practical importance, if any. 



The following tabic? shows the influence of an 



iron tube surrounding a straight iron or copper 

 wire compared with compound wires : — 



WIRES IN IRON TUBE, EACH 1 METRE IN 

 LENGTH. 



Comparative electromotive 

 1 force of the extra cur- 

 rents. 



Approximate comparative 

 increased resistance dur- 

 ing the variable period 

 (that of the stable period 

 being taken as 1.) 



Copper 'wire 2 millimetres diameter, 





1.08 





100 



Same wire insulated in the interior of the 





450 



10.34 



Same joined in the tube at both ends 



275 



10.00 



Same in contact with the tube throughout 









200 



7.83 



Compound wire (copper interior with 









325 



4.35 



Soft Swedish iron, 2 millimetres diameter, 









400 



2.28 



Same wire insulated in the interior of the 





433 



2-78 



Same joined to the tube at both ends.. . . 



240 



2.70 



Same in contact with the tube throughout 









215 



2.60 



Compound wire (steel interior, copper 







107 



1.20 







This table shows that the iron tube has a much 

 greater effect on the copper wire than on the iron 

 wire, the effect in both cases being at its maxi- 

 mum when the tube is insulated from its central 

 conducting wire ; for, while the wire is in con- 

 tact with its tube, there is evidently a shunt 

 action, or eddy current, between the outer coat- 

 ing and the central portion. This Professor 

 Hughes has measured by means of a telephone 

 between the wires and its sheath, and also be- 

 tween two concentric sheaths. "When the sheath 

 is joined to the wire at both ends, the electro- 

 motive force of the extra current is reduced, 

 but the resistance during the variable period is 

 little altered. If, however, as in a coated wire, 

 the wire and sheath are in contact throughout, 

 there is a marked decrease in this resistance. 

 Thus Professor Hughes is of opinion that the 

 shunting effect takes place locally and probably 

 transversely. The passage of an electrical cur- 

 rent then takes place with less opposing resist- 

 ance from self-induction than would be the case 

 if there were no internal partial neutralization 

 of the extra currents. 



ORIGIN OF FAT IN ANIMALS. 



Since the researches of Dumas, Milne-Edwards, 

 and others on insects, and those of Persoz and 

 Boussingault on geese, it has been established that 

 the animal organism has the power of elaborating 

 fatty matters. It was formerly believed that such 



