a current the jar will not only receive but will retain
a charge equal in pressure to that of the device sending the current.
And when you go even farther and bring the terminals near together,
the quick discharge that takes place creates an electric spark which
is in reality a series of alternating flashes that come so fast as to
be blurred into what appears to be one. Could we separate these
flashes we should find that each of them lasts less than a thousandth
part of a second. The frequency of such oscillations is regulated by
what is technically termed capacity, that is the size of the Leyden
jar. The smaller the capacity the greater the frequency of the
flashes.

"Now this spark, or oscillatory discharge emitted from the Leyden jar,
does not result from a single traveling of the current all in one
direction; instead the electricity moves back and forth, or
alternates, and the space where the discharge takes place (and which,
by the way, can be lengthened or decreased as pleases the operator) is
known as the spark gap."

"But I should think this explosion of the spark would make a noise,"
commented Walter.

"Bully for you, little brother!" returned Bob, smiling at His
Highness. "You are quite an electrician. If the current is strong, or,
in other words, if the discharge is a high frequency one, it does.
Hence something has to be used to deaden the sound just as a muffler
is used on a motor boat. It is important, however, that this muffler
should not prevent the operator from watching the condition of his
spark for otherwise he could not keep track of his battery or know
whether it was on the job or not. So you will find little peepholes
of mica or glass in the sides of the muffler."

"Windows," murmured Nancy grasping the idea and translating it into
the vernacular.

"Exactly," Bob agreed. Evidently his audience were understanding what
he was trying to make clear to them.

"Now we have our high frequency oscillations occurring in the spark
discharged from the Leyden jar and jumping the spark gap; nevertheless
they would not do us any good were there not some way to use and
regulate them. This brings us to the induction coil of which I spoke a
second ago."

"It sounds very terrible," smiled Mrs. Crowninshield.

"It isn't, though," answered Bob, returning the smile. "In fact it is
a very simple device--nothing more than a dozen or so twists of copper
wire reeled about a wooden frame exactly as strands of t