ST. JOHN. — WAVE LENGTHS OP ELECTRICITY. 



221 



Professor Oliver J. Lodge remarks as follows in his " Modern Views 

 of Electricity" (page 101, 1889) : "I might go on and say that iron 

 makes an enormously worse conductor than copper for rapidly alter- 

 natintr currents. So it does for currents that alternate with moderate 

 rapidity — a few hundred or thousand a second — like those from a 

 dynamo or telephone ; but, singularly enough, when the rapidity of 

 oscillation is immensely high, as it is in the Leyden jar discharges and 

 lio-htning, iron is every bit as good as copper, because the currents 

 keep to the extreme outer layer of the conductor, and so practically 

 do not find out what it is made of." 



And again in more detail on 

 page 46 of his " Lightning 

 Conductors and Lightning 

 Guards" (1892) we find the 

 following: "But every one 

 will say — and I should have 

 said before trying — surely 

 iron has more self-induction 

 than copper. A current going 

 through iron has to magnetize 

 it in concentric cylinders, and 

 this takes time. But experi- ''' 

 ment declares against this view j 

 for the case of Leyden jar \. 



-o o- 



-o a- 



B 



discharges. Iron is experi- 



L 



Fig. 1. 



mentally better than copper. 

 It would seem, then, that the 



flash is too quick to magnetize the iron ; or else the current con- 

 fines itself so entirely to the outer skin that there is nothing to 

 magnetize." 



The experiment given to substantiate this conclusion is that of the 

 alternate path, as shown in Figure 1. The Leyden jars are charged 

 by an electrical machine, and when a spark occurs at A the charges 

 on the outer coatings may combine by sparking across B or flowing 

 around L. For the path L was used a strip of tinfoil 21 feet long 

 and 3 inches broad, in one case zigzagged backwards and forwards 

 with paraffine paper insulation to abolish self-induction as far as 

 possible, and in the other case wound upon a glass tube to produce 

 as much self-induction as possible. 



When the path L was made by the tinfoil zigzag, the critical dis- 

 tance at B, when sparks sometimes passed and sometimes failed for a 



