382 WRITINGS OF JOSEPH HENRY. [1855- 



Froin the foregoing in regard to the direct discharge, we 

 think the danger to be apprehended from the electricity 

 leaving the wire and striking a person on the road is small. 

 Electricity of sufficient intensity to strike a person at the dis- 

 tance of twenty feet from a perfectly insulated wire would 

 in preference be conducted down the nearest pole. It will 

 however in all cases be most prudent to keep at a proper 

 distance from the wire during the existence of a thunder 

 storm, or even at any time when the sound of thunder is 

 heard in the distance. 



In case of wires passing through cities and attached to 

 houses they should be provided at numerous points with 

 electrical conductors to carry off the discharge to the earth. 

 These consist of copper wires intimately connected with the 

 earth by means of a plate of metal at the lower end, extend- 

 ing up the pole or side of the house, and terminating in a 

 flat plate above, parallel to another plate of metal depending 

 from the wire of the telegraph. The two plates are separated 

 by a thin stratum of air, or some other non-conducting ma- 

 terial, through which the intense discharge from the clouds 

 will readily pass and be conducted to the earth, while the 

 insulation of the wire for the purposes of the telegraph is 

 un-impaired. 



There are other electrical phenomena connected with the 

 telegraph which, though frequently annoying to the operator, 

 are not attended with the same degree of danger to his per- 

 son. These are immediately referable to induction at a 

 distance, and consist entirely in the disturbance of the natu- 

 ral electricity of the wire. Suppose a thunder cloud to be 

 driven by the wind in such a direction as to cross at right 

 angles, for example, the middle of a long line of telegraph 

 wire. During the whole time the cloud is approaching the 

 point of its path directly above the wire, the repulsion of the 

 redundant electricity of the former will constantly drive the 

 natural electricity of the latter farther and farther along the 

 line, so that during the approach of the cloud a continuous 

 current will exist in each half of the line. When the centre 

 of action of the cloud arrives at the nearest point of the wire 



