Q Amateur - 

 ElGctrician 



^d Wii'Gless Operator 



Use of a Permanent Magnet with 

 the Audion 



MANY articles have been published in 

 electrical publications regarding the 

 use of permanent magnets in connection 

 with vacuum tube detectors. Some authors 

 state that the intensity of 

 signals is greatly increased, j^ 

 by the use of magnets, n 

 while others find that no 

 advantage is gained by their 

 use. Experiments with the 

 various types of magnets and 

 several makes of tubes, has 

 shown that while good results 

 may be had without the mag- 

 net, providing a filament rheo- 

 stat with a very delicate ad- 

 justment is used and patiently 

 adjusted, the permanent magnet 

 is an accessory worthy of a 

 place upon the operating table. 

 Because the output of the 

 filament battery varies gradu- 

 ally, especially if a small storage 

 battery is used, readjustment of 

 the rheostat is necessary from time to time. 

 The best adjustment of some tubes is so 

 critical that it can seldom be reached with 

 the standard battery rheostat. 



If the rheostat adjustment is left slightly 

 below the critical best point and a perma- 

 nent magnet (a bar magnet is best) upon 

 a suitable stand is moved toward the tube, 

 the electron discharge wall be varied by the 

 influence of the magnetism, and a point 

 will be reached where the signal intensity 

 of certain stations is loudest. A further 

 movement of the magnet toward the tube 

 will cause a reduction in signal intensity, 

 and sometimes the rheostat adjustment 

 may be such that a second position of 

 stronger intensity may be found with the 



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Convenient stand 



to support magnet 



? in various positions 



magnet very close to the tube. The posi- 

 tion of the magnet in the second field must 

 be more precise than in the first field. It 

 seldom permits more than 1/16 in. varia- 

 tion. The polarity of the magnet will also 

 be found to make a difference, one pole 

 bringing in louder signals than the other 

 pole, regardless of their positions. Inter- 

 ference may often be re- 

 duced by the use of the 

 magnet in the first sensi- 

 tive field, and the ad- 

 justment is far better than that 

 secured by the finest manipula- 

 lation of the receiving instru- 

 ments. A slight movement of 

 the magnet within this field will 

 generally cause one of two sta- 

 tions, having about the same 

 signal intensity, to fade, thus 

 making the other readable. An 

 adjustment of the magnet in the 

 opposite direction may cause 

 the other station to fade and 

 make tne first readable. 



The magnet seems to have a 

 different efifect upon some sta- 

 tions, which though quite loud 

 and readable with the magnet in one of the 

 fields, wall come in still more strongly when 

 the magnet is withdrawn from the tube. 



A convenient stand for supporting the 

 bar magnet in various positions is shown in 

 the illustration. The base of the instru- 

 ment should be cast and turned out of 

 babbitt metal, and may be nickel or silver 

 plated. A hole is drilled and tapped to 

 receive the threaded end of a brass or copper 

 rod, of a length depending upon the height 

 of the detector from the table. A switch- 

 blade clip removed from an old switch, or 

 its equivalent made from spring brass, is 

 soldered to the upper end of the rod and is 

 used to hold a small permanent bar magnet 

 as shown. — H. W. Offins. 



3fi7 



