300
��'Popular Science Monthly
��of a single turn. Reference to Fig. 4 shows that each step to the left of switch 6^ adds in circuit ten turns of the coil, and that these large jumps of inductance may be filled in by the smaller steps se- cured by moving switch i to the right.
���Fig. 5. Arco and Rendahl high power variometer
This switching system has come into wide use within the past few years.
One of the difficult problems of radio engineering is the construction of an easily varied inductance capable of car- rying such high currents as are encoun- tered in the oscillation circuits of power- ful transmitting stations. Flat spiral coils have proved useful, but if they are to be used for final adjustment some way is needed to change their induct- ance gradually without interfering with the current through them. 1915 patent No. 1131187, issued to G. von Arco and R. H. Rendahl, shows an interesting way of doing this. Referring to Fig. 5, two sets of flat spiral coils a and h are seen to be mounted in a framework which permits the gr'oup a to be moved away from the others by swinging them about the pivots / as an axis. The whole set of coils is connected in series, with taps taken off at terminals k, r.nd the units are carefully insulated from each other. When the moving group is in the posi- tion of closest coupling to the fixed coils the system has its maximum inductance, since the magnetic fields of all the coils
��are co-operating; when, however, the h coils are swung out into the position in- dicated by the dotted lines at the bottom of Fig. 5, the maximum addition of fields no longer occurs and the induct- ance of the system is very much re- duced. The special advantages of this method of mounting arise from the fact that parts having large differences of po- tential are kept well separated. Al- though the simple two-coil variometer construction used in receiving coils will give an inductance variation as large as 1 to 15 when insulation difficulties are small, in the two coil form as applied to high-powered transmitter the coils have to be kept so far apart that the maxi- mum inductance is only about twice the minimum. With the sub-divided form shown in this patent, however, heavy currents can be carried and yet a con- siderable inductance variation attained. Fig. 6 shows an interference prevent- er arrangement patented in 1915 by T. B. Miller, specification No. 1127368. In the ordinary interference preventer of Fessenden two primary circuits con- nected to the antenna act on two oppos- ing secondaries ; one primary is adjusted to receive the desired signals selectively and to impress them upon the detector, while undesired signals are caused to af- fect both branches of the circuit equally and oppositely and so produce no final effect. The circuit of Fig. 6 differs from this earlier arrangement in that a single antenna primary circuit is used with two secondaries and two detectors,
���Interference preventer
��and the neutralization of interfering sig- nals is accomplished by opposing their effects in the telephone circuits.
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