Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

.58 RADIO BROADCAST ADVERTISERTJour AerialisSubject toLightningInduction7/iis Qrrester Carries$100-JREE INSURANCESafetvjDemands aBRACHStormKingARRESTERBECAUSE~It Gives MaximumProtectionItWill Not GroundItWill Not AffectReceptionIt Carries $100Free Insurance120 Years Standard on\I America's LeadingRailways\OVER 2,000,000 RADIOSARE PROTECTED BYBRACH ARRESTERSBracliRadio ProductsLS. BRACH MFG. CO..NEWARK.N.J.TORONTO, CAN.Equipment forConstructorthe Home*How to Use Some of the New Equipment Tested andApproved by the " Radio Broadcast " LaboratoryINTERFERENCEBy THE LABORATORY STAFFELIMINATORSas well as the natural sort ofMAN-MADE, "static," will hinder the reception of radiosignals. Fortunately, however, the former can beeliminated in almost all cases. Any kind of electricalspark will set up high-frequency oscillationssimilar to thpse used for broadcasting, the differencebetween the two being that the electricalspark from a motor or other household applianceis of an intermittent and varying intensity, whichresults only in noise in the receiver. There aremany classes of electrical apparatus which cancause interference, such as oil burners, batterychargers, violet ray apparatus, etc. The commutatortype of motor commonly used in connectionwith household appliances is one of the mostcommon offenders. The spark takes place at thecommutator, but the high-frequency oscillationsmay be carried along wires for a block or moreand blanket reception over a large area.FIG.IGround frarolMoloiA combination of fixed condensers and chokecoils, placed in the electric line near the offendingapparatus, will usually eliminate, or at leastgreatly reduce, the interference from this source.The common connections of such a filter apparatusare shown in Fig. i. A choke coil isplaced in each leg of the line supplying the motoror other apparatus. These coilshave a high impedance to highfrequencyoscillations, and are usuallyof approximately 200 millihenriesinductance. They shouldbe wound with wire heavy enoughto carry the current requiredwithout undue heating.l Chokesof this character may be constructedby winding 175 turns ofNo. 16 d.c.c. on a porcelain tubeI inch in diameter and 12 incheslong. The condensers act as a bypathto ground for the high-frequencycurrent. The value of thesecondensers may be from I to 2microfarads, depending upon thenature of the interference. Sometimesanother condenser, shownby the dotted lines, is placedacross the line side of the filter.These condensers should have ahigh enough working voltage totake care of any voltage whichmay be impressed on them. THEThere are several interferencefilters on the market, two ofwhich are shown in the photograph.While the circuit diagram given may notbe exactly that used in the commercial type offilter, these latter are all based on the samegeneral principle. In each device there are fivewires leading out of the case, two of which go tothe electric line, two to the motor or other apparatus,and the fifth to the grounded frame ofthe motor. The Tobe Interference Filter No. I ismade by the Tobe-Deutschmann Company, ofCambridge, Massachusetts, and sells for $15.00.This filter is designed for household appliancesand will work effectively on motors up to J horsepower,and may be used on d.c. or a.c. lines. Itis understood that these filters may be obtained,by special order, for installations of as high as5Oo-kilowatts and 1000 volts potential. The Day-Fan Electric Company, of Dayton, Ohio, alsomake a filter, known as the "Quietus," which isobtainable in two models, one, the No. 6001, forgeneral use in the home, which sells for $10.00,and another, the No. 6003, for use with a householdlighting plant, such as the Delco-Light,which is priced at $8.00.RESISTORSFOR SOCKET-POWER DEVICESRESISTORS designed for use in socket-powerdevices may be of the wire-wound type orof the metallic filament type, but in every case,they must be designed to dissipate the heat generatedfast enough so that they will not burn out.Some manufacturers rate their resistors accordingto how much current may be carried withsafety, while others rate them in watts. To choosethe correct resistance when the rating isgiven incurrent, the amount of current which is to flowin the circuit must be known. This may be measuredor it may be calculated by Ohm's Law. Forinstance, if the resistance is 1000 ohms and thevoltage 100, the voltage divided by resistancewill give the amount of current which will flow,which in this case is o. 10 amperes (100 milliamperes).The current-carrying capacity of the resistorshould therefore be rated at 100 or moremils. In the case of the resistor rated in watts,the procedure is slightly different. The wattage isfound by multiplying the voltage by the current,or in the case of the above resistor, the voltage(100) times the current (o. 10) gives us 10 watts.That is, a looo-ohm resistor having a rating of10 watts would carry the 100 milliamperes with-RADIO BROADCAST Photograph"QUIETUS" AND TOBE-DEUTSCHMANN INTER-FERENCE FILTERSout overheating. Another way in which the carryingcapacity may be figured is by the formula inwhich watts are equal to the current squaredtimes the resistance. Transposing the formula weget:where I is the current, w is the watts rating, andR is the resistance. Substituting the actual values