30S J. Trowbridge — Electrical Waves on Iron Wires. 



ment of the oscillations of the standard time spark. At first 

 thought it seemed a comparatively simple matter to arrange 

 a suitable time circuit. In Hertz's electrical waves,* 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 electro-magnet, 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 the circuit 

 of iron or copper wire which was under examination. I ex- 

 pected thus to obtain a photograph of the spark on my standard 

 circuit at the same instant as that of the spark on the trial cir- 

 cuit of iron or copper. To my surprise I found that the two 

 induction coils did not respond at the same instant to the 

 impulse in the electro-magnet, when the spark gaps were of the 

 same length. It was necessary to make a careful adjustment 

 of these lengths and to modify the amount of capacity in the 

 two circuits. This want of isochronism may have been due to 

 irregularities in the hard rubber condensers which I employed 

 as condensers. This, however, does not seem probable. The 

 condensers were made of sheets of hard rubber ■§■ of an inch 

 in thickness, covered with tin foil, and the set of condensers 

 in the time circuit did not differ geometrically appreciably 

 from the set on the trial circuit. The electrical disturbances 

 on such a connected system is evidently a complicated one 

 when its various reactions are considered ; and the statement 

 given by Hertz, which I have quoted, must be modified if 

 there is any capacity in the circuits of the two Ruhmkorf coils 

 which have a common primary. 



The capacity in the time circuit was the same geometrically 

 as that in the circuit which included the wires under examina- 

 tion. A suitable amount of self-induction was placed in the 

 time circuit. To ascertain whether the time circuit could be 

 relied upon, I made many measurements of the ratio of the 

 oscillations in the time circuit to those in the trial circuits 

 which contained copper wires and the same geometrical capacity. 

 I had no reason to suspect a chajige in the self-induction in my 

 time circuit ; a change might occur, however, in the capacity 

 of the India rubber condensers, due possibly to hysteresis or to 



* Electric waves, Dr. Heinrich Hertz, translated by D. E. Jones, B.Sc. 



