of Electrical Discharges. 



261 



short pieces of wire, so as to make a divided circuit of two 

 branches as before. The cross section of the tin-foil conductor 

 on the inner tube was made as nearly as possible equal to that 

 of the outer, so that equality of resistances was secured without 

 introducing materially different lengths of wire into the two 

 branches. The resistances in every case were adjusted to equal- 

 ity by means of a sensitive mirror galvanometer and a Kohl- 

 rausch bridge. 



With this apparatus the magnetic moments of two magnets 

 of equal weight, cut from the same steel rod and tempered 

 together, are almost exactly equal; and the curious phenome- 

 non exhibited in fig. 1, when the magnets exchanged places in 

 the two coils, very nearly disappears. What is perhaps more to 

 the purpose, the lateral distribution of magnetism in the two 

 magnets is practically identical. Two steel rods as before were 

 placed in the magnetizing spirals; and after ten discharges of 

 the Ley den jar, an examination was made by gradually remov- 

 ing the outer shell with nitric acid and determining the mag- 

 netic moment after each removal of about 50 mgs . The two re- 

 sulting curves, shown in fig. 3, are nearly identical. Both 

 show the same superficial reversal of magnetic polarity. In 

 fact, if curve B is rotated clock-wise through a small angle 

 round its highest point it wdl coincide with that of A. The 

 depth to which this reversed magnetism extends depends, there- 

 fore, almost entirely on the coefficient of self-induction of the 

 discharger, the principal conductor 

 of definite capacity being charged 

 to a given potential. When the ] 

 only dissimilarity between the two 

 branches of a divided conductor is 

 that one is placed inside the other, 

 there is no dissimilarity in the 

 quantity or distribution of the 

 magnetism produced in similar 

 coils in the two branches by a 

 discharge through the compound 

 circuit. There is no evidence of 

 a tendency toward surface trans- ' 

 mission. The law of resistance 

 pertaining to dynamic currents 

 applies also to electric discharges. 



The resistance is a function of the area of the cross-section 

 and not of the periphery of the cross-section. 



A practical bearing of the investigation is that there is no 

 sufficient scientific basis for making lightning conductors of 

 large surface ; and that large sectional area in essential to ample 

 conductivity. 



Evanston, 111. 







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