308 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



in comparison with the other. The spark, however, w^ith the long 

 wire and compound battery is not as brilliant as with the single bat- 

 tery and the short riband coil. 



20. When the shock is produced from a long wire, as in the last 

 experiments, the size of the plates of the battery may be very much 

 reduced, without a corresponding reduction of the intensity of the 

 shock. This is shown in an experiment with the large spool of wire (10). 

 A very small compound battery was formed of six pieces of copper 

 bell wire, about one inch and a half long, and an equal number of 

 pieces of zinc of the same size. When the current from this was 

 passed through the five miles of the wire of the spool, the induced 

 shock was given at once to twenty-six persons joining hands. This 

 astonishing effect placed the action of a coil in a striking point of view. 



21. With the same spool and the single battery used in the former 

 experiments, no shock, or at most a very feeble one, could be obtained. 

 A current, however, was found to pass through the whole length, by 

 its action on the galvanometer; but it was not sufficiently powerful to 

 induce a current which could counteract the resistance of so long a 

 wire. 



22. The induced current in these experiments may be considered 

 as one of considerable intensity^ and small quantity. 



23. The form of the coil has considerable influence on the intensity 

 of the action. In the experiments of Dr Faraday, a long cylindrical 

 coil of thick copper wire, inclosing a rod of soft iron, was used. This 

 form produces the greatest effect when magnetic reaction is employed ; 

 but in the case of simple galvanic induction, I have found the form of 

 the coils and helices represented in the figures most effectual. The 

 several spires are more nearly approximated, and therefore they exert 

 a greater mutual influence. In some cases, as will be seen hereafter, 

 the ring form, shown in Fig. 4, is most eflfectual. 



24. In all cases the several spires of the coil should be well insulated, 

 for although in magnetizing soft iron, and in analogous experiments, 

 the touching of two spires is not attended with any great reduction of 

 action; yet in the case of the induced current, as will be shown in the 

 progress of these investigations, a single contact of two spires is some- 

 times sufficient to neutralize the whole effect. 



