ou can shop around
to advantage. For telephone receivers I would buy a really good pair
with a resistance of about 2500 ohms. Buy also a small mica condenser of
0.002 mf. for a blocking condenser. Your entire outfit will then look as
in Fig. 112. The switch _S_ is a small knife switch.

To operate, leave the switch _S_ open, place the primary and
secondary coils near together as in the figure and listen. The tuning is
varied, while you listen, by moving the slider of the slide-wire tuner
or by moving the switches if you have connected your coil for that
method. Make large changes in the tuning by varying the switch
_s_{1}_ and then turn slowly through all positions of _s_{2}_,
listening at each position.

[Illustration: Fig 112]

When a signal is heard adjust to the position of _s_{1}_ and
_s_{2}_ which gives the loudest signal and then closing _S_
start to tune the secondary circuit. To do this, vary the capacity of
the condenser in the secondary circuit. Don't change the primary tuning
until you have tuned the secondary and can get the signal with good
volume, that is loud. You will want to vary the position of the primary
and secondary coils, that is, vary their coupling, for you will get
sharper tuning as they are drawn farther apart. Sharper tuning means
less interference from other stations which are sending on wave lengths
near that which you wish to receive. Reduce the coupling, therefore, and
then readjust the tuning. It will usually be necessary to make a slight
change in both circuits, in one case with switch _s_{1}_ and in the
other with the variable condenser.

As soon as you can identify any station which you hear sending make a
note of the position of the switches _s_{1}_ and _s_{2}_, and
of the setting of the condenser in the secondary circuit. In that way
you will acquire information as to the proper adjustments to receive
certain wave-lengths. This is calibrating your set by the known
wave-lengths of distant stations.

After learning to receive with this simple set I should recommend buying
a good audion tube. Ask the seller to supply you with a blue print of
the characteristic[11] of the tube taken under the conditions of filament
current and plate voltage which he recommends for its use. Buy a storage
battery and a small slide-wire rheostat, that is variable resistance, to
use in the filament circuit. Buy also a bank of dry batteries of the
proper voltage for the plate circuit of the tube. At t