120 



THE WORLD'S ADVANCE 



Wiring Diagram of a Simple Marconi Multiple 



Tuner, Showing Relation of Different Windings and 



Condensers. 



circuits, and is tuned simply by means 

 of a variable condenser which has a con- 

 siderable range of capacities. This de- 

 sign of tuner is quite selective, and yet 

 is more quickly and more readily ad- 

 justed than the somewhat more selective 

 Fessenden type. Waves impinging upon 

 the aerial, even though highly damped, 

 produce in the low resistance intermedi- 

 ate circuit very feebly damped oscilla- 

 tions, and these can be tuned quite sharp- 

 ly in the detector circuit. 



The general diagram of this type of 

 tuner is as follows: 



The first circuit consisting of the 

 loading coil, condenser and primary of 

 the first loose-coupler is adjusted to re- 

 spond to the desired wave length. The 

 intermediate circuit is then adjusted, 

 after which the detector circuit is brought 

 to the same period as the rest. 



The most important factors in the con- 

 struction of receiving outfits are the 

 proper proportioning of inductances to 

 capacities in order that the amount of 

 dead-ended wire may be reduced to a 

 minimum, the reduction of distributed 

 capacity, the elimination of sliding con- 

 tacts, and the use of coils whose natural 

 periods differ considerably from the 

 wave lengths for which they are to be 

 used. Moreover, in inductive tuners the 

 primary and secondary coils must with 

 the closest coupling be far enough 

 apart to minimize electrostatic capacity 

 effects between the windings. 



The following instructions will enable 



the construction of an outfit on the order 

 of the multiple tuner, but possessing cer- 

 tain advantages over the above-men- 

 tioned tuner in that it is adapted for use 

 with various forms of detectors. The 

 box is to be made of mahogany, prefer- 

 ably, although walnut is also suitable. It 

 is suggested that the box be made with 

 a hinged top in order that the apparatus 

 may be more e*asily assembled. Retain- 

 ers are glued to the sides and bottom of 

 the box in order to strengthen the joints. 



After assembling, the box should be 

 thoroughly sandpapered in order to re- 

 move all traces of glue. It should then 

 be given a coat of mahogany stain, or 

 walnut stain if the latter wood is used. 

 The builder may use a water stain and 

 apply the desired color, after trying vari- 

 ous depths of color on a piece of wood. 



W hen dry, the surface should be light- 

 ly sandpapered to remove any roughness 

 where the stain may have "raised" the 

 wood. A coat of shellac is then applied 

 and when dry it is sandpapered sufficient- 

 ly to bring out a smooth surface. This 

 is given a coat of light varnish. When 

 dry, the surface is sandpapered and given 

 a second coat of varnish. When the lat- 

 ter is dry, the builder should obtain a 

 piece of burlap, some "rubbing oil," an:! 

 the finest powdered pumice stone possi- 

 ble. The burlap is soaked with the oil 

 and sprinkled with the pumice stone to 

 serve in rubbing down the wood. This 

 process requires a couple of hours, but 

 if carefully done it will yield a finish to 

 the box similar to that of a piano. Be- 

 fore sandpapering, it is necessary to make 

 sure that the coats of stain or shellac or 

 varnish are thoroughly dry. Great care 

 should be exercised in sandpapering to 



Forming Block on Which the Cardboard or Sheet 

 Fibre is Placed. 



