244 WRITINGS OF JOSEPH HENRY. [1846 



ON THE RELATION OF TELEGRAPH LINES TO LIGHTNING. 



(Proceedings of the American Philosophical Society, vol. iv, pp. 260-268.) 



June 19, 1846. 



[A letter from S. D. Ingham to Dr. Patterson was read, 

 detailing cases in which the telegraph wires were struck by 

 lightning, and asking the attention of the Society to some 

 interesting questions connected with the mode in which the 

 wires may be affected by electricity.] 



Professor Henry, to whom the letter was referred, made 

 the following report: 



The action of the electricity of the atmosphere on the wires 

 of the electrical telegraph is at the present time a subject of 

 much importance, both on account of its practical bearing, 

 and the number of purely scientific questions which it in- 

 volves. I have accordingly given due attention to the letter 

 referred to me, and have succeeded in collecting a number 

 of facts in reference to the action in question. Some of these 

 are from the observations of different persons along the 

 principal lines, and others from my own investigations dur- 

 ing a thunder storm on the 19th of June, when I was so 

 fortunate as to be present in the olhce of the telegraph in 

 Philadelphia, while a series of very interesting electrical 

 phenomena was exhibited. In connection with the facts 

 derived from these sources, I must ask the indulgence of the 

 Society in frequently referring, in the course of this com- 

 munication, to the results of my previous investigations in 

 dynamic electricity, accounts of which are to be found in the 

 Proceedings and Transactions of this Institution. 



From all the information on the subject of the action of 

 the electricity of the atmosphere on the wires of the telegraph, 

 it is evident that effects are produced in several different ways. 



1. The wires of the telegraph are liable to be struck by a 

 direct discharge of lightning from the clouds, and several 

 cases of this kind have been noticed during the present 

 season. About the 20th of May the lightning struck the 



