St. John — Wave lengths of Electricity on Iron Wires. 323 



where L is the total length of circuit ; A a constant depending 

 only on the form of the circuit, or I A is the inductance of a 

 similar copper circuit ; /j. the permeability ; R the ohmic resist- 

 ance ; p= 27m where n is the number of complete oscillations 

 per second. 



The value of ^=2^ = 36xl0 7 



R for iron wire (diameter 0' 1186 cm ) = -132S ohms per meter. 



" " " " 0-08847 = -227 " " 



" " " " 0- 0785 = -301 " " 



For iron (diameter 0-1186 cm ) 



L'=l-034L = ^A+4/^\ 



L + -034 L=L + Z4/— t 

 T 2 pi 



Calculating the value of L for a similar copper circuit I units 

 long, substituting the value in the above equation, and solving 

 for the three cases we get 



For the iron wire, diameter 0* 1186 cm , ju=430 

 " " " 0-08847 /* = 389 



" " " 0- 0785 //=336 



These values for the permeability all fall within a reasonable 

 limit and have for an average ^ = 385. These are the values 

 found for different specimens of wire made by the same com- 

 pany, but the specimens were wound and unwound and 

 stretched many times during the series of observations. 



Besides the shortening of the wave length there is shown a 

 decided increase in the damping as has already been observed 

 by Trowbridge and Bjerknes. In fig. 4 of the plate the curves 

 for iron fall below the corresponding ones for copper, but 

 owing to the change in the activity of the spark no exact 

 measurement was made. It was only observed that the bolom- 

 eter throws with the copper circuit were always greater than 

 with the iron circuit of the same dimensions, when the spark 

 was constant as far as the eye and the ear could judge. 



A value can readily be calculated for the damping factor 



Rt 



s 2l in the case of the iron and copper. Lord Rayleigh's 

 formula for the resistance under very rapid oscillations is : — 



R'=Vij»^R 



An. Jour. Sci.— Third Series, Vol. XLVIII, No. 286.— October, 1804. 

 21 



