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Show VARIOMETER TUNES TO DIFFERENT STATIONS I1 to Change in Relative Coil Cuts Out One and Brings in Other B I'M L I OODIiEY. Imerloa'fl Poremosi Radio tathorltj That th mere change In relathf position of two colls of wire will actually "cut out " one transnilttlnp radio station and bring In another, is rather surprising. Yet that Is exactly ex-actly what happena when hi- variometer vario-meter In o radio set Is adjusted. All radio circuits must hav two things capacity and Inductance. The capacity in usually composed of a condenser; the indfetance of a number f splraled turns of wire Variable inductances are of three kinds those known as slide wire inductances, in-ductances, wherein a sliding contact moves up and down the coll. makln? contact with each turn of wire; step-by-step Inductances where a switch makes contact with certain predetermined predeter-mined points along the coil, or ho variometer M (.M T1( CHANG! In the variometer, Inductance is varied by a change In tho magnetic relationship of its two colls I'suallv one coll revolves within the other and the two are connected In "series." When their planes coincide, and the direction of winding on both stationary sta-tionary nud movable oils Is the same, the magnetic fields produced by currents cur-rents flowln gin them also coincide. The inductance of the combination Is 1 then at a maximum. Should the movable coll be resolved re-solved ISO degrees so that the planes of the two colls again coincide, but so that the direction of the winding I upon the coils Is In opposition, cur-' cur-' rent flowing through the comblna-! comblna-! Hon will produce magnetic effects which oppose each other Since the coils are exactly equal and since the current flowing In them Is als iii.il the magnetic effects ' will be equal If the coils were so j close together as to actually merge , one Into the other, the Inductance of I the combination would be zero. It Is mechanically impossible to j build u variometer so that the colls : actually do merge. Many designers have made the effort, however, to i keep the two sets of colls ery lose together in order that a maximum range mav be had CAPACITY. In addition to hiving Inductance all coils have capacity. In the best variometers a very careful balance between a minimum o fcapaclty and I a maximum of inductance variation Is made so that the maximum of, wave length variation may be had I That Is the Important thing. If the rolls are too close toglH i i ipaclty of the ceH hi' i n will completely offset njB cr-aso in the range of InductM variation. The diagram above is a crosdB tion of a variometer, and "-'howajJJ! method Of varying th'' lnduct.incdT opposition or i "incidence of the nfr netl fields of both colli -j |
