to 'Electricity and Magnetism. 205 



arrangement is given vvlien the contact is formed, but it is 

 very feeble in comparison with the other. The spark, how- 

 ever, with the long wire and compound battery is not as bril- 

 liant 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 experi- 

 ment with the large spool of wire (10.). A very small com- 

 pound 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 con- 

 sidered as one of considerahle 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 sim- 

 ple 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 ex- 

 ert 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 insulated ; for although in magnetizing soft iron, and in 

 analogous experiments, the touching of two spires is not at- 

 tended 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. 



25. It must be recollected that all the experiments with 

 these coils and helices, unless otherwise mentioned, are made 

 without the reaction of iron temporarily magnetized ; since the 

 introduction of this would, in some cases, interfere with the 

 action, and render the results more complex. 



