While on this subject we may add that these coils will produce sparks
from the two ends of the wire of the secondary coil. These sparks vary
in length according to the amount of wire in the coil. Small ones are
made which give a spark a quarter of an inch in length, while others
are made which will give sparks 10, 12, and 16 inches in length. In the
latter, however, there are many miles of wire in the secondary coil.
The largest induction-coil known is one which was made for an English
scientist. There are 341,850 turns, or 280 miles, of wire in the
secondary coil. With 30 cells of Grove battery this coil will give a
spark 42 inches in length. You may form some idea of the effect of this
induction-coil when we state that if we desired to produce the same
length of spark direct from batteries, without using an induction-coil,
we should require the combined volts of pressure of 60,000 to 100,000
cells of battery.
Having explained to you briefly the induction-coil--how it is made
and its action--we must ask you to bear these principles in mind, and
presently we will tell you how it is used in the telephone.