﻿442 Prof. John Trowbridge on the Change of Period 



trical form — the electrostatic capacity of the two circuits being 

 the same. 



With reference, therefore, both to the theory of electrical 

 waves and to the theories of magnetism, it seems important 

 to determine whether there is a lengthening of electrical waves 

 on iron wires. 



The method of investigation I employed was the same as 

 that which I have described in my paper on the damping of 

 electrical waves on iron wires. A leyden jar was discharged 

 through the given wires and the resulting spark, spread out 

 by a revolving mirror, was photographed. The apparatus was 

 also modified as I have described in my paper on " Electrical 

 Resonance and Electrical Interference " *. 



It was important in this investigation to be able to compare 

 the times of oscillation on the iron circuits and the copper 

 circuits, and it seemed best to employ some method of imprint- 

 ing, so to speak, the time of a standard circuit on each photo- 

 graph, beside the photograph of the spark produced on either 

 the iron circuit or the copper circuit; for if the speed of the 

 mirror changed, this change could be readily detected by the 

 measurements of the oscillations of the standard time-spark. 

 At the first thought it seemed a comparatively simple matter 

 to arrange a suitable time circuit. In Hertz, l Electric 

 Waves ' f, appendix p. 271, it is stated : — 



" Let the primary coils of two induction-coils be placed in 

 the same circuit, and let their spark-gaps be so adjusted as to 

 be just on the point of sparking. Any cause which starts 

 sparking in one of them will now make the other begin to 

 spark as well; and this quite independently of the mutual 

 action of the light emitted by the two sparks, which, indeed, 

 can easily be excluded. " 



I therefore slipped two induction-coils of exactly the same 

 self-induction on a long electromagnet, placing them sym- 

 metrically upon it. In the circuits of these induction-coils I 

 placed the same amount of capacity. The spark-gap on the 

 time circuit was made of the same length as that on a circuit 

 of iron or copper wire which was under examination. I 

 expected thus to obtain a photograph of the spark, on my 

 standard circuit at the same instant as that of the spark on the 

 trial circuit of iron or copper. To my surprise, I found 

 that the two induction-coils did not respond at the same in- 

 stant to the impulse in the electromagnet, when the spark-gaps 

 were of the same length. It was necessary to make careful 

 adjustment of these lengths and to modify the amount of 



* Phil. Mag. Aug. 1894. 



f ' Electric Waves,' Dr. Heinrich Hertz, translated by D. E. Jones, B.Sc. 



