1838] WRITINGS OF JOSEPH HENRY. Ill 



12. Also a small horseshoe 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 re- 

 sults mentioned in the paper except those with a Cruick- 

 shank trough, can be obtained with one or two batteries of 

 the above size, particularly if excited by a strong solution. 

 The manner of interrupting the circuit of the conductor by 

 means of a rasp /;, 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, 

 discovered 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 elec- 

 tricity of the metal, disturbed for an instant by the induc- 

 tion of the primary clirrent. Before proceeding to the other 

 parts of these investigations, it is important to state the re- 

 sults of a number of preliminary experiments, made to de- 

 termine more definitely the conditions which influence the 

 action of the spiral conductor. 



15. When the electricity is of low intensity, as in the case 

 of the thermo-electrical pile, or a large single battery weakly 

 excited with dilute acid, the flat ribbon coil No. 1, ninety- 



