Some form of switch for quickly connecting the aerial and ground to
either the transmitting or receiving apparatus is necessary.
Low powered stations using an induction coil as a transmitter will find
a 250-volt double pole, double throw porcelain base switch to be
suitable. Such a switch is connected up according to the diagram shown
in Fig. 70. As clearly illustrated, the receiving apparatus is in use
when the switch is thrown to the right, and the transmitter is ready for
operation when the switch is in the left-hand position. The ease and
speed with which such a switch may be thrown from right to left or vice
versa may be much facilitated by fitting it with a larger and longer
handle than the one usually provided.
[Illustration: Fig. 70. Connections for Aerial Switch.]
An ordinary double pole, double throw switch should be mounted on the
wall within easy reach of the operator, while the two types of switch
about to be described are best situated directly on the operating table
alongside of the transmitting key.
All insulated parts of an aerial switch such as the base, insulating
bar, handle, etc., should be made of some good insulating material, as
porcelain, hard rubber or ebonite. All insulation which is used in power
switches such as slate, wood, marble, etc., is worthless and should not
be used.
If a loop aerial system is used, a switch is necessary, which will break
the primary transmitting current so that in case of an accidental touch
of the key while receiving, the high voltage discharge will not pass
across the anchor gap into the receptor and badly damage the same or
shock the operator. The connections of such a switch and the loop aerial
are shown in the wiring diagram of the De Forest system.
[Illustration: Fig. 71. Aerial Switches.]
A switch may be easily constructed from a 50 ampere, single throw,
triple pole, fuseless power switch. If it is mounted on a slate base
remove it and set it up on a base of the same size made of fiber or
porcelain. Flat unglazed tiles of various sizes are obtainable from tile
setters at a very low cost. They may be easily bored with an ordinary
steel twist drill. Such tiles are excellent insulators when not exposed
to moisture and are useful for bases, etc., in many cases. A heavy coat
of shellac varnish will make the tile impervious to moisture.
A hard rubber rod, _S_, 3 inches long and 5/8 inch diameter, is mounted
on both sides of the base in alignment with the contacts. A strip of
hard rubber, _Y_, 1 inch wide, 1/2 inch thick and 6 1/2 inches long is
fitted with two contacts similar to those mounted on the base of the
switch. The contacts are mounted at a distance apart equal to the
distance between the outside knife blades of the switch. A 1/8-inch hole
is bored through the strip at both ends, through the axis of the rods
along their entire length and through the base. A piece of brass rod 4
inches long and threaded at both ends with an 8-32 die is passed through
the holes and yoke, so that the rods may be held firmly to the base and
the strip to the tops of the rods by two nuts screwed on the ends of the
brass rod.
[Illustration: Fig. 72. Detail of Contacts.]
The upper contacts should be directly over the two outside lower ones.
They are made from a strip of brass 1/2 inch wide, 3 1/2 inches long and
1/16 inch thick (bent as in Fig. 72). A binding post is mounted on the
yoke directly above and connecting with each one of the contacts.
The middle pole of the switch is connected to the primary of the coil or
transformer, so that when the switch is down, the primary circuit is
completed, and when it is up it is broken.
[Illustration: Fig. 73. Details of Switch Parts.]
Some may prefer to make the complete switch, and in that case the
dimensions given in Fig. 73 may be of aid. The knife blades are 6 inches
long, 5/8 inch wide and 1/8 inch thick. Three are required. A 1/8-inch
hole is bored 3/8 inch from one end, and the other end is cut out in the
shape shown in the illustration. An insulating bar 5 x 3/4 x 5/8 inches
is made of hard rubber and three notches are cut in the back face, one
directly in the center and one 2 inches on either side.
[Illustration: Fig. 74. Method of Fastening Knife.]
Three 1/8-inch holes are bored one in the center of each groove and at
right angles to it. An 8-32 machine screw 3/4 inch long is placed in the
notch in the end of the knife blade and the screw passed through the
hole in the insulating bar. The blade may then be clamped tightly in
place by putting a nut on the end of the screw. The screw which holds
the middle knife blade in place is longer than the other two so that a
handle may be fitted on.
The bearings are made the same size and shape as the contacts
illustrated in Fig. 72, with the exception that a 1/8-inch hole is bored
through near the top so that a small bolt may be slipped in to hold the
knife blades in place.
The base of the switch measures 8 x 9 x 3/4 inches. The parts are
assembled as in Fig. 71.
Another type of aerial switch called a "T" switch is illustrated in the
same figure. A piece of 3/4-inch hard rubber is sawed in the shape of a
T, 3 1/2 inches high and 6 1/2 inches wide at the top. The upper
contacts are mounted on arms of the "T" and connected to binding posts
placed immediately above them, A slot, 1/2 inch wide and 3 inches long,
sawed in the center of the leg of the T, permits the middle knife blade
to move without obstruction.
[Illustration: Fig. 75. "T" Aerial Switch.]
A double pole, double throw switch may be easily adapted to form a "T"
switch. Two knives are cut from sheet brass, of the same shape as those
already on the switch but twice as long. The contacts at one end of the
porcelain base are removed and the bearings of the knife blade fastened
there in their stead. This forms a single throw, double pole switch
having knife blades twice the length of the original double pole, double
throw switch. A "T" shaped support for the upper contacts may be sawed
out of 1/2-inch hard rubber and smoothed up with a file. The support is
fastened upright by means of two screws which pass through holes in the
center of the base where the knife bearings originally were. The two
contacts which were removed from the base are fastened to the support
directly over the knife blades so that they make a good contact when the
switch is up. Two binding posts mounted above the contacts make
connection with them.
If desirable the switch may be fitted with a third knife blade and
contact so that when the switch is up in position for receiving, the
primary circuit of the induction coil or small transformer is open.
It is obvious that the dimensions, etc., need not be the same as those
given here but may be adapted to suit the size and design of the switch
which is to be altered. The knives should always be as long as
convenient and the contacts separated by a wide space.
[Illustration: Fig. 76. "United" Wireless Lightning Switch.]
[Illustration: Fig. 77. Shoemaker Tuning Coil and Aerial Switch.]
Figs. 76 and 77 illustrate the aerial switches used in the installations
of the United Wireless Telegraph Company. These illustrations give a
good idea of the long knife blades employed. The lightning switch is
fitted with a micrometer spark gap on top of the switch which is in
service when the handle is up. The small spark gap affords a path for
the lightning to jump into the ground, but cannot be bridged by the
receiving currents.
*Anchor Gaps.*—When a loop aerial system is employed for both
transmitting and receiving, a device known as an anchor gap becomes
necessary.
[Illustration: Fig. 78. "United" Wireless Anchor Gaps.]
The construction and connection of a simple anchor gap are shown in Fig.
79. A ring 1 3/4 inches outside and 1 1/4 inches inside is cut out of
5/8-inch hard rubber or fiber and smoothed up. Three 1/8-inch holes are
bored in the periphery of the ring at 120 degrees to each other. The
holes are threaded with a 10-24 tap. Three small binding posts are each
fitted with a 3/16-inch brass rod 7/8 inch long, having a 10-24 thread
so that they may be screwed into the holes in the ring.
[Illustration: Fig. 79. Anchor Gap.]
The gap between the ends of the two rods connected to the aerial and the
receiving apparatus is adjusted until it is about 1/8 inch long. The
third point is screwed in until it all but touches the other two.