In designing an apparatus to put on board a boat or a balloon, where
weight is a consideration of primary importance, there is again a
difference. There are three things that come into play--iron, copper,
and electric current. The current weighs nothing, therefore, if you
are going to sacrifice everything else to weight, you may have
comparatively little iron, but you must have enough copper to be able
to carry the electric current; and under such circumstances you must
not mind heating your wires nearly red hot to pass the biggest
possible current. Provide as little copper as you conveniently can,
sacrificing economy in that case to the attainment of your object;
but, of course, you must use fireproof material, such as asbestos, for
insulating, instead of cotton or silk.