This is called the inductance coil. You will notice that the wire is
covered with cotton except for this little strip of wire extending
lengthwise where I've scraped the cotton off with sandpaper so as to
accommodate the sliding contacts. These sliding contacts can be made
from curtain rings with holes punched in them, through which are
passed copper rivets. These rivets press against the bare path of
the coil and can be moved to and fro until you find the exact point
where your set is in tune with the sending station."
CHAPTER VII
IN THE DARK
"Now," continued Dr. Dale, as he glanced round the circle of eager
faces, alight with interest in the subject, "we're getting pretty
close to the time when one picks up the receiver and begins to listen
in.
"But as the electric vibrations, if left alone, would have a good deal
of trouble in passing through the telephone receiver, we must have a
condenser to help them out. This is very easily made by gluing a piece
of tinfoil about one and a half inches square to each side of a sheet
of mica. Then you must have two strips of tinfoil, one extending from
each side of the mica. If you haven't any mica, a sheet of ordinary
writing paper will do, though the mica is better.
"The telephone receiver you will have to buy, as a satisfactory one
can't very well be made by an amateur.
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