Page:Popular Science Monthly Volume 90.djvu/333

 Popular Science Monthly

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��the sparking-surfaces are made, and also upon the freedom with which air is admitted to the gap. Copper plates which are not provided with airtight gaskets soon become so thickly covered with oxides or compounds including nitrogen that gap action grows irregular; it is therefore desirable to exclude air from such instruments. When silver surfaces are used, the oxidization is not so rapid, and it is not necessary to take such precautions.

The drawing, which is taken from 1916 patent No. 1,171,598, issued to L. De- Forest, shows a method which has been found very useful for making quenched- gaps airtight. The structure comprises the usual plates 4, piled upon each other and clamped by means of screw 16 into containing frame 13. Electrical con- nection is made through the wires 18 which are connected to the clamping- heads 15. The novelty claimed for the present patenc consists in the use, between the individual plates of the quenched-gap, of washers consisting of soft compressible varnished or inipreg- nated fabric. When this material is clamped between the plates it effectually prevents the entrance of air, and so limits the formation of oxides upon the surfaces of the plates. Further air- tightness may be secured by filling the space between the peripheral rims of the plates with a compound such as paraffin or beeswax, as shown at 10.

In building quenched-gaps of airtight construction, care must be taken that the normal cooling of the gap is not greatly reduced by prevention of ventilation. Also, in using compressible gaskets, the screw 16 must be carefully adjusted in order that the plates be not forced into accidental contact. If the necessary precautions are taken to insure per- manence of separation of the plates, an airtight gap is likely to require cleaning much less frequently than one in which oxidization is permitted to progress freely.

Such an airtight quenched spark gap is especially useful on ships at sea and at any land stations which would be effected by the rapid action of salt air upon the material of the gap where it is heated by the spark.

In such a case a gap of this type will not only work more efficiently but will wear considerably longer.

��Spark Sender for Sustained Wireless Waves

��THE drawing shows a transmitter described in U. S. Patent 1,166,892, issued in 1 91 6 to R. A. Fessenden. The high-voltage direct current dynamo A charges the outer plates of the condenser

B, through the resistance or impedance

C. The condenser discharges through the oscillatory circuit formed by the adjustable inductances in wires D and E, and the rotating spark-gap F. This gap consists of a stationary elec-

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���Fessenden's method for coupling the os- cillatory circuit with that of the aerial

trode placed near to a disk which revolves at high speeds; the inventor suggests that the relative movement of the two sparking points may be as high as 12 miles a minute. The construction may involve two rotating disks, or some other mechanical form which will operate at very great speeds and so prevent the formation of any ark in the gap. In such an arrangement, the specification states, continuous and regular discharges are produced and sustained, or practically sustained waves are generated at high efficiency.

The mode of coupling to the antenna- to-ground circuit through the internal plates G, H of the sending condenser possesses some interest. This is equiva- lent in many ways to the static couple transmitters which have recently been tried in connection with various types of ark and spark generators. In order to secure good transfer of oscillating current energy from closed to open circuit it is necessary that the radiating circuit be in tune with the local circuit formed by the plates of B and the inductance in the lead wires D, E. This is accomplished by the usual insertion of variable inductance and capacity in the antenna circuit.

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