1846] WRITINGS OP JOSEPH HENRY. 249 



stant effect of this kind indicates a constant accession of elec- 

 tricity at one part of the wire, and a constant discharge at tlie 

 other. 



,3. The natural electricity of the wire of the telegraph is 

 liable to be disturbed by the ordinary electrical induction of a 

 distant cloud. Suppose a thunder-cloud driven by the wind 

 in such a direction as to cross one end of the line of the 

 telegraph at the elevation — say of a mile; during the whole 

 time of the approach of the cloud to the point of its path 

 directly above the wire, the repulsion of the redundant elec- 

 tricity with which it is charged would constantly drive more 

 and more of the natural electricity of the wire to the farther 

 end of the line, and would thus give rise to a current. When 

 the cloud arrived at the point nearest to the wire, the current 

 would cease for a moment; and as the repulsion gradually 

 diminished by the receding of the cloud, the natural elec- 

 tricity of the wire would gradually return to its normal state, 

 giving rise to a current in an opposite direction. If the 

 cloud were driven by the wind parallel to the line of the 

 telegraph a current would be produced towards each end of 

 the wire, and these would constantly vary in intensity with 

 the different positions of the cloud. Although currents pro- 

 duced in this way may be too feeble to set in motion the 

 marking apparatus, yet they may have sufficient power to 

 influence the action of the current of the battery so as to 

 interfere with the perfect operation of the machine. 



4. Powerful electrical currents are produced in the wires 

 of the telegraph by every flash of lightning which takes 

 place within many miles of the line, b}^ the action of dynamic 

 induction; which differs from the action last described in 

 being the result of the influence of electricity in motion on 

 the natural electricity of the conductor. The effect of this 

 induction, w^hich is the most fruitful source of disturbance, 

 Avill be best illustrated by an account of some experiments 

 of my own, presented to the Society in 1843. A copper wire 

 was suspended by silk strings around the ceiling of an upper 



