332 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



the distance. In these cases, however, the change always takes place 

 at a very small distance from the conducting wire ; and in this respect 

 the result is similar to the effect of a primary current from the dis- 

 charge of a small jar. 



121. The most important experiments, in reference to distance, 

 were made in the lecture room of my respected friend Dr Hare of 

 Philadelphia, with the splendid electrical apparatus described in the 

 Fifth Volume (new series) of the Transactions of this Society. The 

 battery consists of thirty-two jars, each of the capacity of a gallon. A 

 thick copper wire of about yVth of an inch in diameter and eighty feet 

 in length, was stretched across the lecture room, and its ends brought 

 to the battery, so as to form a trapezium, the longer side of which was 

 about thirty-five feet. Along this side a wire was stretched of the 

 ordinary bell size, and the extreme ends of this joined by a spiral, simi- 

 lar to the arransjement shown in Fig. 13. 



' c ' The two wires were at first placed within 



the distance of about an inch, and after- 

 wards constantly separated after each dis- 

 charge of the whole battery through the 

 thick wire. When a break was made in 

 the second wire at a^ no magnetism was 

 developed in a needle in the spiral at &, but 

 when the circuit was complete, the needle 

 at each discharge indicated a current in the 

 ^ same direction as that of the battery. When 



c place of the battery, b spiral. thc distancc of thc two wircs was iucrcascd 

 to sixteen inches, and the ends of the second wire placed in two glasses 

 of mercury, and a finger of each hand plunged into the metal, a shock 

 was received. The direction of the current was still the same, but the 

 magnetism not as strong as at a less distance. 



122. The second wire was next arranged around the other, so as to 

 enclose it. The magnetism by this arrangement appeared stronger than 

 with the last; the direction of the current was still the same, and con- 

 tinued thus, until the two wires were at ^very point separated to the 

 distance of twelve feet, except in one place where they were obliged 

 to be crossed at the distance of seven feet, but here the wires were 



