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Page:Popular Science Monthly Volume 89.djvu/641

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How to Become a Wireless Operator

II. — Construction of a One- Mile Wireless Transmitter

By T. M. Lewis

(Continued from September issue)

��IN AN article published last month, directions were given for putting together a little buzzer wireless telegraph set which would operate over a distance of a few hundred feet or even more. This small outfit was sufficient to demonstrate such of the principles of wireless telegraphy as should be known by ever>' student and to send messages- from one house to another nearby. The receiver was sensitive enough to pick up messages from commercial stations for some distance around, provided that a fairly long antenna wire was connected to it and properly tuned.

The amateur who has built and tested the buzzer set will want next to own and operate an outfit with which he can signal over greater distances. It is the purpose of this article to describe the construction of a wireless telegraph sender which can be made cheaply and easily, and which will give good strong signals at a suitable receiving station located as much as a mile or more away. The apparatus for the receiver will be taken up in later articles; the experimen- ter may well spend the intervening time in building his sender.

Transmitting Coil One of the first requisites in increasing the distance over which messages can be sent is to increase the effective power of the sender. The buzzer run from a couple of dry cells is not strong enough to make waves which will carry ver>- far, so it becomes necessary to get an instru- ment which will do better. Such an apparatus is the ordinary induction or spark-coil. The amateur may build his own spark-coil by following the descrip- tions which are given in a great man\- books on experimental electricity, but in the long run he will find it cheaper and more satisfactory to buy one. An automobile jump-spark coil is about as good a small induction coil as can be obtained. Often it is pos.sible to get one

��at a nominal price from a garage or an electrician in the neighborhood. Even if purchased new from an electrical supply house, a good coil capable of giving a i-in. spark between needle points in air will not cost more than three or four dollars.

There is also needed a Morse key, for sending the dots and dashes which make up the signal letters. This may be an ordinary' telegraph key, which costs about seventy-five cents, or e\'en a "strap" or signal key of the kind that sells for only twenty-five or thirty cents. If he desires, the experimenter may build his own key as shown in last month's article. For the heavier currents used in the spark-coil (as compared to the buzzer) it is a good plan to use larger key-contacts than those illustrated. They may be made by soldering copper washers on each of the contact screws.

To furnish power for the coil, the best thing is a 6 or 8-volt storage-battery.

���Twelve dry cells arranged so as to distribute the load between the two sets

This is quite expensive, however, and also requires occasional recharging. Satisfactory results may be secured by using 12 dr>' cells connected as shown in Fig. I. With the battery arranged in this way the voltage is no greater than can be had from 6 cells, but the load is distributed between two sets of cells working side by side in parallel. As a result, the battery will last much longer than if onh- 6 cells were used. The vibra- tor on the spark-coil should be adjusted so that it buzzes freely, with a high- pitched sound, whenever the sending key is pressed. A spark-gap connected

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