THE keys used in wireless work differ from those used in commercial wire
telegraphy in being much heavier and stronger, so as to conduct and
break the heavier currents without heating. Where very powerful currents
must be broken the contacts of the key usually operate under oil or in
the magnetic field of a pair of electromagnets. In other cases a small
key is used to operate a heavy relay. In the former type a metal arm
projects downward from the lever of an ordinary Morse telegraph key into
a compartment filled with oil. The arm is provided with a platinum point
which makes contact with another similar point on the bottom of the
compartment, so that the break is made under oil and the key remains
cool and does not arc. In the second form the same result is
accomplished by a magnetic "blowout." The "blowout" is merely an
electromagnet connected in series with the key and its poles at right
angles to the contacts of the key. When the circuit is broken the
magnetic field extinguishes the arc which tends to form between the
contacts of the key.
An ordinary Morse key may be used with coils operated on batteries if
they do not give more than a four-inch spark. Larger coils or
transformers, especially if they are operated on the 110-volt current,
require a heavier key. Where an ordinary Morse key is used with a 3-inch
or 4-inch coil it is a very good plan to provide an auxiliary conductor,
one end of which is fastened to the metal base and the other end to the
adjusting screw on the back of the lever. A piece of flexible lamp cord
is suitable for this purpose. This precaution saves the pivots of the
key from heating and possible burning by heavy currents.
[Illustration: Fig. 67. Morse Key fitted with Extension Lever.]
The most convenient method is to fit up an old Morse key in the manner
shown in Fig. 67. The lever is extended by a piece of aluminum or brass
6 inches long and 5/16 inch thick. The exact shape and the dimensions
are indicated in the illustration.
The extension lever is provided with a 1/2-inch round brass or nickel
steel contact 7/8 inch long. The lower end of the contact is bored and
tapped to receive a short machine screw. The other contact is a similar
piece projecting downwards from a small arm fastened to the top of a
standard 1/2 inch diameter and 2 inches long. The arm is a strip of
brass 1/2 inch wide, 1/4 inch thick, and 1 1/2 inches long, fastened to
the top of the standard with a machine screw. The brass standard is held
in an upright position by means of a small machine screw which passes
through the base. A wire connects with this screw to a heavy binding
post mounted on the base. Connection is made with contact on the
extension lever by means of a piece of heavy flexible lamp cord, which
also runs to a binding post mounted on the base. The contact is
insulated from the extension lever by means of two mica insulating
washers and a bushing. Avoid running the connection to the base of the
key so that the current must pass through the bearings. The contacts
should be filed until they are perfectly flat and square across and make
contact over their entire surface. When carefully constructed a key of
this type will carry without heating almost any currents in reach of the
amateur operator and which his induction coil or transformer will stand.
[Illustration: Fig. 68. Wireless Key.]
The plans and dimensions for a heavier key are shown in Fig. 68. A
wooden pattern of the base and bearings is made and taken to a brass
foundry where a casting in brass may be secured. The pattern should be
of the same shape and size as the finished article represented in the
figure. It is given a slight taper so that it may be easily withdrawn
from the sand mold. The brass casting is finished up square and smooth
by grinding on an emery wheel or by careful filing. The lever is a piece
of brass rod 1/2 inch square and 6 1/2 inches long. One end is fitted
with an ordinary hard rubber Morse key knob. The knurled nuts and
thumbscrews are of the same size as those used on an ordinary key, and
may be purchased from an electrical supply house.
[Illustration: Fig. 69. "United Wireless Type Key."]
The pivot is a piece of 3/16 inch round steel 1 inch long, and passes
through the lever 1 1/2 inches from the rear end. The ends are ground or
turned to sharp points and rest in bearings formed by boring a recess in
the ends of the thumbscrews. The thumbscrews pass through the center of
the bearing, standards 3/4 inch above the base. The lever is clamped to
the center of the pivot by means of a small machine screw. A heavy piece
of spring brass, _S_, 1/2 inch wide and 3 inches long, is fastened to
the base at one end with a short 8-32 screw. The other end is bent
upward so that it presses hard against the lower end of an adjusting
screw, which passes through the lever 1 1/4 inches forward of the pivot.
This spring serves as an auxiliary conductor and saves the key from
carrying an excessive current through its bearings. By adjusting the
thumbscrew, the tension of the key may be regulated to suit the
operator. The contacts of the key are 1/8-inch round nickel steel alloy
or pure silver, set in short 1/4-inch machine screws which fit into
correspondingly threaded holes in the base and the lever. This makes the
contacts easily removable for renewal. The lower contact is set in a
longer screw than the upper one so that connection may be made to it. It
is insulated from the base by a hard rubber brushing.
The contacts must be directly above one another and in perfect alignment
or else they will pit and stick.