to Electricity and Magnetism. 203 



formed of five hundred and forty-six yards of wire, ^^gth of 

 an inch in diameter, the several spires of which are insulated 

 by a coating of cement. Helix No. 5 consists of fifteen hun- 

 dred yards of silvered copper wire y^^jth of an inch in dia- 

 meter, covered with cotton, and is of the form of No. 4. 



10. Besides these I was favoured with the loan of a large 

 spool of copper wire, covered with cotton, -^-^ih. of an inch 

 in diameter, and five miles long. It is wound on a small 

 axis of iron, and forms a solid cylinder of v/ire, eighteen 

 inches long, and thirteen in diameter. 



11. For determining the direction of induced currents, a 

 magnetizing 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 horseshoe is frequently referred to, which 

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

 |ths 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 develope 

 the magnetism of the iron. The instrument is used for indi- 

 cating 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 cop- 

 per, 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 men- 

 tioned in the paper, except those with a Cruickshanks trough, 

 can be obtained Vith 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, 

 b, is shown in the same figure. 



Section I. — Conditions isohicJi iiifluence the inductioti of a Cur- 

 rent 07i itself. 



14-. The phaenomenon 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 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 experiments, made to determine 



