205 
to 'Electricity and Magnetism, 
arrangement is given when 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 
v/ell 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. 
