Page:Popular Science Monthly Volume 90.djvu/969

 Popular Science Monthly

��953

��The effect of the added condensers is to reduce the effective resistance of the aerial circuit. In the old lay-out all of the an- tenna current passed through the arc itself, and this of course occasioned large resistance losses. In the new arrangement the larger part of the radiation current passes through the shunt condenser, which has a much lower effective resistance, and the wasted power is greatly reduced. By shunting the condenser i with a circuit containing inductance and resistance in the proper proportions, the impedance of the arc circuit is still further increased and still more of the main antenna current forced through the shunting condenser. This causes still further gains in efficiency.

With a 5 k.w. arc the antenna current has been increased lOO per cent by making the condenser i equal to and that of the condenser 2 twice the capacity of the antenna. Using a 50 k.w. generator, the current has been pushed up 74 per cent by suitable adjustment of the condensers, and then additionally increased 11 per cent by adding the shunting circuit of inductance and resistance. It is important to keep the inductance of the leads 4 and 5 very small. It would appear that circuits of this sort should be of considerable value in arc working, for not only is the efficency of the system markedly increased but the tuning of the system is made much sharper and difficulties due to the radiation of extraneous wavelengths are cut down.

��Making a Tuning Coil Slider with a Good Contact

IN making up wireless receiving equip- ment it is often necessary' to have a slider which moves easily and yet makes a good electrical contact. The following de- scribes a type which has been used with satisfaction. A piece of sheet-brass 1/32 in. thick and ^ in. wide is needed. The length depends on the size of the slider rod, but the dimensions given are for rods 5/16 in. square.

Cut the brass strips to size and bend the square corners 5/32 in. on each side of the middle. Bend two other square corners 5/32 in. from each end. This gives a rec- tangular tube 5/16 in. wide inside measure- ment and about j.'^ in. high. There is left a 3/16 in. space after the rod is in place.

Place the seam at the top and solder a brass strip 5/16 in. wide and i ^ in. long, lengthwise of the tube. Bend the ends as

��shown in the sketch so that they will exert considerable pressure on the rod when it is inserted in the tube. Drill a hole for a small machine screw through the spring and tube.

On the opposite side solder a phosphor- bronze strip which should be bent, as

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��A slider made of sheet brass and a spring to make perfect contact with the coils of wire

shown, to make contact with the winding of the primar>\ The length of this strip and its width at the point of contact will have to be determined by the builder. It should be long enough, however, to provide sufficient pressure and still not catch on the turns of the coil. For a handle the hard rubber grip from the generator crank of a magneto telephone is satisfactory' and may be attached to the slider by means of a machine screw and nut, the screw passing through the hole which has been already drilled in the tube. — D. R. Lewis.

���A Simple Experimental Spark- Gap in a Case

ABASE, A, is turned out of wood on a lathe and then given two or three coats of varnish. The inside diameter of this base should be just large enough to p)ermit the glass bell jar J? to fit snug- ly in it.

An ordinary spark-gap is mounted on the base and the connections brought through the base and sealed to prevent the leakage of oil.^

Enough good transformer-oil C is then poured into the base to cover it K"'"- thick, and the bell-jar is placed in position. This makes the gap air-tight and, by reducing oxidation, aids in producing a humming tone in the receivers.

��A spark-gap inclosed in an air-tight case

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