on the Wires of the Electric Telegraph. 191 



within many miles of the line, by the action of dynamic in- 

 duction ; which differs from the action last described, in being 

 the result of the influence of electricity in motion on the na- 

 tural electricity of the conductor. The effect of this induction, 

 which is the most fruitful source of disturbance, will be best 

 illustrated by an account of some experiments of my own, 

 presented to the Society in 1843. A copper wire was sus- 

 pended by silk strings around the ceiling of an upper room, so 

 as to form a parallelogram of about sixty feet by thirty on the 

 sides; and in the cellar of the same building, immediately be- 

 low, another parallelogram of the same dimensions was placed. 

 When a spark from an electrical machine was transmitted 

 through the upper parallelogram, an induced current was de- 

 veloped in the lower one sufficiently powerful to magnetize 

 needles, although two floors intervened, and the conductors 

 were separated to the distance of thirty feet. In this expe- 

 riment no electricity passed through the floors from one con- 

 ductor to the other; the effect was entirely due to the repul- 

 sive action of the electricity in motion in the upper wire on 

 the natural electricity of the lower. In another experiment, 

 two wires, about 400 feet long, were stretched parallel to each 

 other between two buildings; a spark of electricity sent 

 through one produced a current in the other, though the two 

 were separated to the distance of 300 feet ; and from all the 

 experiments, it was concluded that the distance might be in- 

 definitely increased, provided the wires were lengthened in a 

 corresponding ratio. 



That the same effect is produced by the repulsive action of 

 the electrical discharge in the heavens, is shown by the fol- 

 lowing modification of the foregoing arrangement. One of 

 the wires was removed, and the other so lengthened at one 

 end as to pass into my study, and thence through a cellar 

 window into an adjacent well. With every flash of lightning 

 which took place in the heavens, within at least a circle of 

 twenty miles around Princeton, needles were magnetized in 

 the study by the induced current developed in the wire. The 

 same efiFect was produced by soldering a wire to the metallic 

 roof of the house, and passing it down into the well ; at every 

 flash of lightning a series of currents in alternate directions 

 was produced in the wire. 



I was also led, from these results, to infer that induced cur- 

 rents must traverse the litie of a railroad, and this I found to 

 be the case. Sparks were seen at the breaks in the continuity 

 of the rail, with every flash of a distant thunder-cloud. 



Similar effects, but in a greater degree, must be produced 

 on the wire of the telegraph by every discharge in the heavens; 



