212 Contrihutio7is to Electricity and Magnetism. 



and five miles long. It is wound on a small axis of iron, and 

 forms nearly a solid cylinder of wire, eighteen inches long, and 

 thirteen in diameter. 



11. For determining the direction of induced currents, a mag- 

 netizing spiral was generally used, which consists of about thirty 

 spires of copper wire, in the form of a cylinder, and so small as 

 just to admit a sewing needle into the axis. 



12. Also a small horse-shoe is frequently referred to, which 

 is formed of a piece of soft iron, about three inches long, and 

 fths of an inch thick ; each leg is surrounded with about five 

 feet of copper bell wire. This length is so small, that only a 

 current of electricity of considerable quantity can develop the 

 magnetism of the iron. The instrument is used for indicating 

 the existence of such a current. 



13. The battery used in most of the experiments is shown in 

 Fig. 1. It is formed of three concentric cylinders of copper, and 

 two interposed cylinders of zinc. It is about eight inches high, 

 five inches in diameter, and exposes about one square foot and 

 three quarters of zinc surface, estimating both sides of the metal. 

 In some of the experiments a larger battery was used, weakly 

 charged, but all the results mentioned in the paper, except those 

 with a Cruickshank trough, can be obtained with one or two bat- 

 teries of the above size, particularly if excited by a strong solu- 

 tion. The method of interrupting the circuit of the conductor 

 by means of a rasp, h, is shown in the same Figure. 



SECTION I. 



Conditions which influence the, Induction of a Current on itself. 



14. The phenomenon of the spiral conductor is at present 

 known by the name of the induction of a current on itself, to 

 distinguish it from the induction of the secondary current, dis- 

 covered by Dr. Faraday. The two, however, belong to the same 

 class, and experiments render it probable that the spark given by 

 the long conductor is, from the natural electricity of the metal, 

 disturbed for an instant by the induction of the primary current. 

 Before proceeding to the other parts of these investigations, it is 

 important to state the results of a number of preliminary experi- 

 ments, made to determine more definitely the conditions which 

 influence the action of the spiral conductor. 



