Page:Popular Science Monthly Volume 92.djvu/323

 �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 the 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.

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