St. John — Wave lengths of Electricity on Iron Wires. 321 



252 given above by the last two additions, but differs by 4 0m 

 from the value found when the copper was used. 



The same series of observations was repeated with a second 

 pair of finer wires, diameter of copper wire - 07836 cm , diam- 

 eter of iron wire 0"0785 om . The results are shown in the 

 lower pair of curves in fig. 4 of the plate. A comparison of 

 the curves shows the same general result which appears more 

 distinctly from the following table : 



1st Maxima. 2d Maxima. 3d Maxima. 4th Maxim. 



Cu. Fe. Difference. Cu. Fe. Difference. Cu. Fe. Difference. Cu. Fe. Diff. 



Upper pair 45 



42-5 



2-5 



306 



301 



5 



5625 553 



9-5 



818 805 13 



Lower pair 40 



37-5 



2-5 



300 



294 



6 



552 540 



12 



799 784 15 



The successive differences should be in the ratio of 1, 3, 5, 7 if 

 the theory of the present investigation is correct. The differ- 

 ences for the first two maxima are very small, so that the ex- 

 perimental error in their determination would be relatively 

 large, and in the case of the fourth maximum the damping was 

 so great that it was difficult to fix the point with certainty. 

 The difference for the third maximum was relatively large and 

 the determination of the point was sharp. Taking this differ- 

 ence as a point of reference, the calculated and observed values 

 are shown in the accompanying table : 



1st Maxima. 2d Maxima. 3d Maxima. 4th Maxima. 



Calcu- Ob- Calcu- Ob- Calcu- Ob- Calcu- Ob- 



lated. served, lated. served, lated. served. lated. served. 



Upper pair 1-9 2-5 5-7 5 9-5 9-5 133 13 



Lower pair 2-4 2-5 7"2 6 12 12 16-8 15 



The observed half wave lengths for the four series are : 



Copper (diameter 0- 1201 0m ) 255-6 cm 

 Iron (diameter 0- 1186 ) 25T6 

 Copper (diameter 0-07836 ) 251*6 

 Iron (diameter 0-07850 ) 246'8 



The wires in each pair were as near the same diameter as could 

 be found, the iron of the larger pair having slightly the smaller 

 diameter, but the copper being the smaller one in the second 

 case. In other respects the circuits compared were as nearly 

 identical as possible. The capacity per unit length being the 

 same for wires of the same diameter, the shortening of the 

 wave length when iron displaced copper must be caused by an 

 increased self-induction due to the magnetic properties of the 

 iron. This implies that the magnetization of iron can be 

 reversed 115 million times per second. This reduces the " time 

 lug" of magnetization to very narrow limits, if forces of such 

 duration can magnetize the iron. 



In the case of extremely rajDid oscillations Prof. J. J. Thomp- 

 son has shown (Recent Researches in Electricity and Magnet- 



