192 Prof. Henry on Electrical Induction 



and the phaenomena vvliich I witnessed on the 19th of June in 

 the telegraph office in Philadelphia, were, I am sure, of this 

 kind. In the midst of the hurry of the transmission of the 

 congressional intelligence from Washington to Philadelphia, 

 and thence to New York, the apparatus began to work irre- 

 gularly. The operator at each end of the line announced at 

 the same time a storm at Washington, and another at Jersey 

 City. The portion of the circuit of the telegraph which en- 

 tered the building, and was connected with one pole of the 

 galvanic battery, happened to pass within the distance of less 

 than an inch of the wire which served to form the connexion 

 of the other pole with the earth. Across this space, at an in- 

 terval of every few minutes, a series of sparks in rapid succes- 

 sion was observed to pass ; and when one of the storms arrived 

 so near Philadelphia that the lightning could be seen, each 

 series of sparks was found to be simultaneous with a flash in 

 the heavens. Now we cannot suppose for a moment that 

 the wire was actually struck at the time each flash took place; 

 and indeed it was observed that the sparks were produced 

 when the cloud and flash were at the distance of several miles 

 to the east of the line of the wire. The inevitable conclusion 

 is, that all the exhibition of electrical phagnomena witnessed 

 during the afternoon was purely the effect of induction, or 

 the mere disturbance of the natural electricity of the wire at a 

 distance, without any transfer of the fluid from the cloud to 

 the apparatus. 



The discharge between the two portions of the wire con- 

 tinued for more than an hour, when the effect became so 

 powerful, that the superintendent, alarmed for the safety of 

 the building, connected the long wire with the city gas-pipes, 

 and thus transmitted the current silently to the ground. I 

 was surprised at the quantity and intensity of the current; it 

 is well known, that to affect a common galvanometer with 

 ordinary electricity, requires the discharge of a large battery; 

 but such was the quantity of the induced current exhibited on 

 this occasion, that the needle of an ordinary vertical galvano- 

 meter, with a short wire, and apparently of little sensibility, 

 was moved several degrees. 



The pungency of the spark was also, as might have been 

 expected, very great. When a small break was made in the 

 circuit, and the parts joined by the fore-finger and thumb, the 

 discharge transmitted through the hand affected the whole arm 

 up to the shoulder. I was informed by the superintendent, 

 that on another occasion a spark passed over the surface of 

 the spool of wire, surrounding the legs of the horse-shoe 

 magnet at right angles to the spires; and such was its inten- 



