1838] "WRITINGS OF JOSEPH HENRY. 113 



alone would give but a very feeble shock, was used with 

 helix No. 1, an intense shock was received from the induc- 

 tion, 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 ribbon 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 reduc- 

 tion of the intensity of the shock. This is shown in an ex- 

 periment 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 galva- 

 nometer ; but it was not sufficiently powerful to induce a cur- 

 rent which could counteract the resistance of so long a wire. 



22. The induced current in these experiments may be con- 

 sidered 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 ef- 

 fect 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 effectual. 



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