214 Contributions to Electricity and Magnetism. 



was used with helix No. 1, an intense shock was received from 

 the induction, when the contact was broken. Also a slight shock 

 in this arrangement is given when the contact is formed, but it is 

 very feeble in comparison with the other. The spark, however, 

 with the long wire and compound battery is not as brilliant as 

 with the single battery 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, each 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 places 

 the action of a coil in a striking point of view. 



21. With the same spool and the single battery used in the form- 

 er experiments, no shock, or at most only 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 consid- 

 ered as one of considei^ahle intensity, and small quantity. 



23. The form of the coil has considerable influence on the in- 

 tensity 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 induc- 

 tion, 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 effectual. 



24. In all cases the several spires of the coil should be well in- 

 sulated, 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 



