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 considerable intensity, and small quantity. 
23. The form of the coil has considerable influence onthe i” 
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, {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 7 
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 apy 
great reduction of action ; yet in the case of the induced current, 
as will be shown in the progress of these investigations, a single 
