Page:Popular Science Monthly Volume 89.djvu/963

 How to Become a Wireless Operator

IV. — Simple Adjustments and Connections By T. M. Lewis

{Continued from November Issue)

��IN THE article published last month there were giv^en descriptions of a crystal-detector and stopping-condenser to be made and used in connection with the transmitting set of the October article, for sending wireless messages over a distance of a mile or thereabout. Both the detector and the condenser are of types which can later be used in receiving stations which will pick up the messages from large commercial or government plants not only nearby, but hundreds of miles away. With the small sender using a spark-coil, however, the range will be limited to a mile or so, unless the aerials at both stations are large.

The Test-Buzzer

In using a crystal-detector it is necessary to be able to find out instantly whether or not the adjustment is sensitive. When the needle-point bears lightl)' upon some parts of the crystal, the receiver is sensitive and able to translate messages coming from a distance; with the contact at other points, however, the instruments seem absolutely dead.

Obviously, to be certain that messages can be received effectively, one must be sure that his detector is properly adjusted. The best way to do this, and the way which is used bj' the professional operators in most large stations, is to take advantage of the feeble signal-waves induced by a buzzer. By setting up a small sending- outfit, such as described in the September issue of the Popular Science Monthly in the first article of this series, the sensitive- ness of the detector may be tested by listening in the recei\'ing telephones and at the same time pressing the testing-key.

Figure i shows how to wire up the buzzer, strap-key and dry cell described in the first article. The only difference from the little sender used to signal from one room to another is that the vibrator-contact post of the buzzer is connected to a miniature aerial wire only a foot or two long, instead of to a genuine, full-sized antenna. The miniature aerial is run along the table

��about 2 or 3 in. from one of the wires leading to the detector, as indicated in the illustration. Fig. 2.

The Change-Over Switch

In order to shift connections from sending to receiving, there must be provided a good-sized double-pole double-throw knife- switch. The lever -arms of the switch should be at least six inches long, and the

��8UZZER ^

���TE5T AERIAL

It is necessary to use a test buzzer to find out if the adjustment is sensitive

jaws should be mounted upon a slate, marble, or fiber base a corresponding distance apart. If the switch used is too small it will not have enough insulation to prevent the sparks from the secondary of the induction-coil from jumping to ground by way of the receiving contacts.

A second-hand knife-switch of this size and type can be bought for about one dollar or less; if none can be obtained, it is not difficult to improv-ise from },^ by J-2-in. strip copper, an instrument which will work perfectly. It is only necessary to obser\-e closely the construction of the big knife-switches of the double-throw type, in some central station, and to imitate them as accurately as possible. A number of brief articles have been published in the technical magazines, gixing details of construction and dimensions for such switches. The connections for the change- over switch are shown in Fig. 2.

The Detector-Protecting Switch

When the wireless station is completely equipped with detector and spark-coil, it is essential to make some provision which will protect the delicateh' adjusted cn,-stal from the \Iolent impulses set up by the

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