Lessons in Wireless Telegraphy

by Alfred Powell Morgan

FIG. 65.

Consider the circuit shown in the accompanying illustration. The action of the transformer is to charge the condenser to such a point that the voltage is sufficient to leap the spark gap and cause a discharge. The rush of current which is oscillatory takes place through the condenser, spark gap and primary turns of the helix, or in other words through the closed circuits. The secondary turns of the helix, which are those forming part of the aerial circuit, are larger in number than those of the primary, and because of this ratio cause currents of higher voltage than those of the condenser to be impressed upon the aerial system. The currents in the aerial system surge up and down the aerial through the helix into the ground.

FIG. 66. Receiving Circuits.

A shows a simple receiving circuit wherein a single slide tuning coil is connected to a detector. The high frequency currents generated in the aerial surge up and down the system and pass through the detector on their way to the ground. By moving the slider back and forth the electrical length of the circuit may be varied to suit the length of the incoming waves. Oscillations may be forced upon such a circuit, that is, if the waves are very powerful they will pass through the system and effect the detector no matter whether the slider is adjusted to suit them or not. This would cause interference and confusion in case more than one station were operating at a time.

By adding a second slider and a condenser as shown in B, this may be avoided to a considerable extent, for slider No. 1 may be adjusted to the desired signals and slider No. 2 placed in a position such as will give the branch of the circuit from the aerial, through the coil and into the ground of which it is a part, a period suited to the objectionable wave and so carry off the latter into the ground without effecting the detector. The desired signals will pass into the ground through the other branch of the circuit and operate the detector which lies in their path. The selectivity of the outfit may be further increased by the addition of a variable condenser.

A variable condenser may be placed in one or more of a great many positions. The accompanying illustrations show several. The effect of a condenser placed in series with the ground or aerial is just the opposite of that of a loading coil. It decreases the period and shortens the wave length to which it is adapted.

The Amateur’s Wireless Handy Book shows over one hundred wiring diagrams starting from the simplest and going to the most complicated in a natural sequence.

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