oil,
will transmit nicely up to a distance of one mile; while a 12-in.
coil made on the same plan will transmit 20 miles or even more
under favorable conditions.

Change the coil described, as follows: Insert an ordinary
telegraph key in the battery circuit, and attach two small pieces
of wire with a brass ball on each, by inserting them in the
binding-posts of the coil as shown at B B". Of these two terminal
wires one is grounded to earth, while the other wire is sent aloft
and is called the aerial line. This constitutes all there is to
the sending apparatus.

Now for the receiving apparatus. In the earlier receiving
instruments a coherer was used, consisting of a glass tube about
1/8-in. diameter, in which were two silver pistons separated by
nickel and silver filings, in a partial vacuum. This receiver was
difficult of adjustment and slow in transmission. An instrument
much less complicated and inexpensive and which will work well can
be made thus:

Take a 5-cp. incandescent lamp and break off the tip at the dotted
line, as shown in Fig. 5. This can be done by giving the glass tip
or point a quick blow with a file or other thin edged piece of
metal. Then with a blow-torch heat the broken edges until red hot
and turn the edges in as seen in Fig. 6. Remove the carbon
filament in the lamp and bend the two small platinum wires so they
will point at each other as in Fig. 6, W W. Screw the lamp into an
ordinary wall socket which will serve as a base as in Fig. 7. Make
a solution of 1 part sulphuric acid to 4 parts of water, and fill
the lamp about two-thirds full (Fig. 7). This will make an
excellent receiver. It will be necessary to adjust the platinum
points, W W, to suit the distance the message is to be worked. For
a mile or less the points should be about 1/16 in. apart, and
closer for longer distances.

The tuning coil is simply a variable choking coil, made of No. 14
insulated copper wire wound on an iron core, as shown in Fig. 7.
After winding, carefully scrape the insulation from one side of
the coil, in a straight line from top to bottom, the full length
of the coil, uncovering just enough to allow a

[Illustration: Aerial]

good contact for the sliding piece. The tuning is done by sliding
the contact piece, which is made of light copper wire, along the
convolutions of the tuning coil until you can hear the signals.
The signals are heard in a telephone receiver, which is shown
connected in shunt across