Page:Popular Science Monthly Volume 92.djvu/170

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154
Popular Science Monthly

of the apparatus, it may be outdistancing all other sub-sea methods in range.

The principles upon which ground wireless telegraphy works are interesting. Ordinary electric current conduction is the operating factor, instead of radiation as in radiotelegraphy. The coils of the sending buzzer are connected with two grounds, as shown in the diagram. At every break of the circuit caused by the buzzer armature, the inductive discharge that ordinarily causes a spark at the armature contacts, discharges to the earth. The relative high voltage sends an electric current from one plate to the other.


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It is obvious that we have not merely a narrow conductor between the plates, but a conductor which is as big as the earth! The result is that, while most of the current going from one ground to the other takes the straight-line route, a good part of it spreads out. The lines of flow in reality appear just like the lines of force which are shown by iron filings between two opposite magnetic poles.

Some of these far-spreading streams of current will reach the buried plates of the receiving station. The leads of the receiving station will "tap" these streams — which are highly pulsating - and a telephone will detect them.

This at once explains why the line going through the plates of one station should be parallel to that going through the grounds of the other. Another fact that is found by experience is that the further the grounds of each station are separated, the louder will the received signals be. This second phenomenon can be explained by referring to the diagram. Here an ammeter is connected with a battery through a metal block of high resistance. This block stimulates the action of the ground between the two wireless stations. It is very much like an ordinary shunt that is put across an ammeter. The greater the resistance of such a shunt, the less current will go through it and the more will go through the ammeter.

Now, in placing the grounds of the stations farther apart, we increase the distance across the theoretical block. Hence, the metal that the current must cross, and the metal's resistance will be increased. The ammeter will then receive more current. For just such reasons, when the actual grounds are buried farther apart, the telephones will receive a larger current.

In practice, you should space your grounds at least twenty feet apart, though it would be much better to have them separated over fifty feet. The neatest and most efficient hook-up to start with is shown in the diagram. Here the house lighting mains are utilized, with a bank of lamps to cut down the voltage. The arrangement enables you to use as much current as the size of the buzzer wires will permit. It also enables you to use the grounded side of the mains instead of one which you would otherwise have to make yourself. If your house is not wired, however, as many as a dozen dry cells, or an equivalent storage battery, may be employed. You will have to make two outside grounds for your station by burying a few pipes in the ground.

By providing your key with a third contact, a simple break-in system is obtained. The telephones shown in the receiving circuit are high resistance wireless receivers. In conjunction with all such high resistance telephones, your spark coil should be used as a step-up transformer. That is, the secondary of your coil should be connected with the receivers and the primary with two grounds.

Of course, should you have a good pair of telephones which are of low resistance, you may use them without the spark coil by connecting them directly with the grounds.