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.
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