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Show Value of Proper Coil Connections Knowledge of Lenz' Law Necessary to Understand Phase Selection. By BRUSTEN BRUNN In Radio World. For a proper understanding of the subject of colls and phase relations It Is necessary to know something about Lenz' law. Suppose that a sensitive current meter me-ter be connected in series with a coil of wire. If a bar magnet Is suddenly sud-denly Inserted into this coil and brought to rest there will be a deflection deflec-tion of the current meter, showing that the act of introducing the bur magnet produces a current in the coll. As soon as the magnet coznes to rest the current stops. Now, if the magnet is suddenly withdrawn there will again be deflection of the meter, but this time It is in the opposite direction from the first deflection. The current set np in the coll exerts ex-erts a back force on the bar magnet, that Is, the induced current opposes the motion of the magnet. That is Lenz" law, or an application of it Effect of Second Coll. Now suppose that Instead of a bar magnet we use a second coil which Is carrying a steady current. There is a magnetic field about this coil Just as there Is about the magnet. In fact, the coll carrying the steady current is an electromagnet. When this Is brought up suddenly to the coil which Is connected across the current meter there will be a deflection of the meter. Again when the current-carrying coil Is withdrawn from the other coil there will be a deflection in the meter, but In opposite direction. In this case also the current, induced in the coil and meter by the motion of the steady z FIG. 1 . P lATE CW't.;W!''?1in!!i , t.ijfcji'';!ajiiaaaa- H-j- FIG. 2 The directions of the coil fields and the proper way to connect an RF transformer are shown at left, while at right Is a pictorial representation of the same method of connection. The rule applies where more than one RF transformer is used in a circuit. cir-cuit. current-carrying coil, exerts a back force on the moving coil which Is nl-ways nl-ways opposing the motion. This also comes under Lenz' law. If two coils are placed close together to-gether and If there is a varying current cur-rent In one of them, then there will be Induced a current in the second coll, provided the circuit is closed, and this current will be in such .1 direction as to oppose any change in the current In the first coil. For example, ex-ample, If the current In the first coil Is alternating it Is continuously Increasing In-creasing and decreasing. Secondary's Retarding Effect. The current in the secondary, provided pro-vided that this coll Is closed, also will be alternating, but the current In the secondary will retard the current In the primary. The effect of the current in the secondary is always to oppose any variation in the current in the primary. pri-mary. This also comes under Lenz' law. Whatever may be the cause of the interaction, the current In the secondary sec-ondary will always he such as to keep the magnetic field unchanged. If the magnetic Held in the primary Is Increasing, the current In the sec ondary decreases so as to keep the total magnetic field the same. Likewise, Like-wise, when the magnetic held in the primary decreases, the current In the secondary increases so ns to keep the total magnetic field constant. This applies to tiie case of tae moving bar magnet as we as to th" cases of alternating al-ternating current in tic primary and to a moving electromagnet. As nn illustration of tin's law con-' sider Fig. 1. Thi may well represent j any ordinary radio frequency trans- former h iving two witcTngs, 1.1 and j L2. Suppose that nn ait'TiiatP'g cur- I rent Is lowing in the primary LI, and j that at s .!!) Insdat.t ti.e direc; idi of the oil-rent is such that ih i:i::c:ie?io field may be reprcser.red as M ri'rc-tion ri'rc-tion hy tin- arrow Ml. that Is. Inside ;ho coll. The enrr' t.t in t. priiasry Indiices n current in t.'.e s,..-.,i-.,i.:ry. or I an emf if the cir.nit is open, and tli. dirocti'Wi of tl.'s induced rer-ept or 1 en;f Is s:pdi thai !: .) gta-iie ;; '! I i npp. se ! in that primary. Tip-I arrow, y.'l. !:)'!(:! !'. ii'- 'ion if! ii,o icagpelic fad.', u tie seioi darv ; r.:-; li,'- is , -.. .wPe ; Ml. TV !:.-- I ;i..n of li e cn-rif.ts in t:;e two cops is indirr.t'-d t-y the si. on arrows at-'aciied at-'aciied ci ti.e wlrir.g. Ti.e uirec: ion of the two windings is the same hut the directions f the current are opposite. oppo-site. Fields Are 90 Degrees Apart. From this explanation It must not be taken that the two fields are 180 degrees apart in time. They are only 90 degrees apart. This means thnt when the intensity of the magnetic field or of the primary current Is maximum max-imum that of the secondary is zero, not minimum. At the instant that the primary current is maximum there Is no change In the magnetic field, and hence, no current is induced In the secondary at that instant. Yv"hen the primary current Is zero the magnetic field about it changes most rapidly and then the induced current in the secondary is maximum for that reason. When connecting a radiofrequency or any other transformer, the terminals termi-nals should be connected as shown In Fig. 1. P should go to the plate of the first tube, B should go to the plate battery, F should go to minus C and G to the grid of the succeeding tube. This gives the most stable connection where the circuit contains more than one such transformer. There is also another advantage and that Is that the capacity coupling between the primary and the secondary is the least. Not only is the capacity least but the current cur-rent through this capacity is the least for a given capacity and frequency. The reason for this is that the two terminals which are at the greatest potential difference are farthest apart in space as well. Fine Wire for Primary. An important consideration in the design of radiofrequency coils is the size of wire in the primary winding. Many have thoughtlessly made this of heavy wire in order to reduce the resistance re-sistance in the circuit. This coil may have a' resistance of an ohm or two while it is connected In series with the plate resistance which may be from 5,000 to 50,000 ohms. Obviously nothing can be gained by reducing the resistance of the primary winding. Much, however, can be gained by making mak-ing the primary of fine wire. The finer the wire is in this winding the less will be the eddy currents, which are Induced In It by the current In the secondary. The fine wire in the primary pri-mary will materially increase the selectivity se-lectivity of the secondary, or, in other words, reduce the resistance in the secondary. Still another advantage Is gained by using fine wire, and that is the reduction of distributed capacity between the two windings. The finer the wire In the primary is the less will be the capacity between the windings, since capacity is largely a matter of dimensions. Reducing the capacity will Increase the voltage that Is Induced In-duced in the secondary and hence will increase the sensitivity of Uie set. Still another advantage of fine wire is the space required. Ffity turns of No. 40 wire, say, can be placed on the coil without any appreciable increase in tlie dimensions, whereas the use of No. 20 wire would require increased space even if only a few turns were used in the primary winding. Even if the wire is as fine as No. 40 a good many turns could be put on the wind-lug wind-lug form before the added resistance could be appreciable in comparison with the plate circuit resistance of the tube. |