306 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



ing 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 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 inter- 

 posed 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 Cruickshank 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 Fara- 

 day. 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 induc- 

 tion 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 more definitely the con- 

 ditions 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 riband coil No. 1, ninety-three feet long, is found to 



