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 



SPRING 



-— "i- 



//'- 



/y 



X 



//-'( 



X. 



EM 



PHOSPHOR BRONZE 



^TUBE 



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 



