ElectromagneticBy varying the magneticfield you can produce anelectromotive force capa-ble of suspending anobject in the field or re-

pelling it

By HAROLD STRAND

This repulsion coil packs enough power to propel a thick aluminumcylinder into the oir for a distance of two or more feet.

EXPERIMENTS per formedwith this repulsion coil enableyou to further understand

such electromagnetic principlesas Lenz's law of induced currentand how the number of turns ofwire in a transformer determinethe voltage that will be produced.

Lenz's law holds that an in-duced current is always in sucha direction that the magnetic fieldbuild-up around the conductorwill oppose the magnetic fieldwhich induced it. This principleis shown in Fig. 1 where the mag-netic field built around the coilactually threw the aluminum ringin the air.

The thick-wall aluminum ringacts like a closed-circuit singleturn secondary winding throughwhich the magnetic flux producedby the larger primary cuts. Thisinduces a heavy current in thering and a magnetic field aroundit and since the two fields are inopposition to each other, the re-sult is to throw the ring out ofthe field.

These Repulsion Forces can beobserved by holding the ringdown with your finger when thebutton is depressed. Slowly allowthe ring to raise to its maximumpoint which will be near the topof the core. Pressing down on thering will show how the opposingfields react, because it will takeconsiderable pressure to hold itdown.

When repulsion force equals pull of grav-ity the ring can "f loat" on the magnetic

field.

Repulsion CoilWhen near to the main coil, the

maximum current is developed inthe ring and the resulting mag-netic field around the ring is alsomaximum. The reaction betweenthis strong field and that from themain coil is responsible for therepulsion effect.

Heat produced in the ring whenit is held down shows why elec-trical conductors have to have asuitable cross-sectional area inorder to carry the required cur-rent without overheating. In wir-ing, this is determined by con-sulting a table which gives thesize of wire required to safelycarry a given current withoutheating.

Suspend the aluminum ring onthe magnetic field (Fig. 2) byallowing it to move up the core toa point where the strength of thefield just balances the weight ofthe ling, or the repulsion forceequals the pull of gravity. When itis down near to the main coil andthe maximum magnetic field ispresent, the repulsion force ofreaction effect greatly exceeds thepull of gravity so the ring is vio-lently thrown into the air.

Transfer of Energy from onecoil to another by electromagneticinduction is another experimentthat can be conducted. This coilcan be used to show the principlesof all transformers, where a pri-mary and secondary winding areplaced on a laminated iron core.The relation of the number ofturns on each winding determinesthe voltage that will be producedat the secondary.

Place a portable coil of wirewith a small lamp connected toits ends over the core to act asthe secondary (Fig. 8), with themain coil as the primary. Whenheld near the top of the core thelamp will barely light, but as itis slowly moved down the core,the light increases until it burnsat full candlepower at the bottom(Fig. 9).

The transformer principle worksthis way. When an alternatingcurrent is applied to the primary,an alternating magnetic fluxwhich rises and falls in step with

Notches have to be filed on the end of the plasticcore tube to clear stop pins used to hold the spool end.

the current is developed. This flux cutsthrough the turns of the secondary windingand through the laws of electromagnetic in-duction, a voltage is induced in the second-ary. If the secondary circuit is closed witha load, a current will flow. If the primaryhas 100 turns of wire and the secondary has10 turns, the ratio will be 10:1 or the voltagedeveloped in the secondary will be 1/10 of theapplied voltage or 10 volts, less a small value

for losses and regulation.In commercial transformers the core is

made as a compact unit with a closed circuitfor the flux to be as short as possible. Thisminimizes leakage reactance.

In our experiment the coil has about 800turns, or with 115 volts that is about seventurns per volt. Theoretically this would callfor about 7 x 6.3 or 44.1 turns on the second-ary to light a 6.3-volt pilot lamp. There are,however, certain iron and copper losses andmost important in this case the core does notprovide a closed path for the flux but issimply a bundle of straight iron strips andthe flux has to pass through the air in itspath from the top to the lower end so thecore, as a transformer is very inefficient.Therefore, you have to add more turns tooffset these losses.

From experiment it was found that about78 turns on the secondary will producearound 6 volts to the lamp when the coilwas fully down on the base and only about1 volt while near the top.

This Difference in Voltage is due to thefact that when at the top of the core, themaximum flux lines cannot cut throughthe secondary winding. The flux is weak atthis point so a weak voltage is induced. Onthe other hand, the greatest amount of fluxcan link through the turns when the coil isdown close to the main coil so maximumvoltage is developed. A secondary coil de-signed to be adjustable is the principle of aregulating transformer used for special ap-plications requiring variable voltage.

You can demonstrate the principles ex-plained very easily with your repulsion coilby following the methods described. With alittle ingenuity you should be able to workout other interesting experiments.

When operating the coil, do not hold theswitch button depressed any longer than nec-essary to perform the experiment, becausethe coil may overheat. It is designed to carrythe maximum amount of current it will standin order to provide good repulsion for thealuminum ling. Continuous use would causean overload on the winding. If the coil be-comes quite warm after a number of experi-ments, allow it to cool a while before con-tinuing.

Start Construction (Fig. 3) by making upthe iron core. Cut enough pieces of 1/2-in.-wide and 6-in.-long soft sheet metal (see Ma-terials list) so that when clamped tightlytogether they make a stack about 9/16 in.thick. You can use almost any soft steel,except galvanized iron or turned sheet steeland the thickness is not too important. I usedstock that was 1/32 in. thick.

Clamp the stock together and drill 1/8-in.holes for three iron rivets (Fig. 3). Also drillthe two small holes used for pin stops thathold the plastic coil spools. Round the cor-

96 SCIENCE nnd MECHANICS

ners so a piece of 3/4-in. insidediameter (id) plastic tubing(Fig. 3C) can be fitted overthe core.

For spool ends, mark off two2-1/4in squares on 1/8in plastic(Fig. 3A) with a sharp-pointedtool and cut to size. Positioncore on plastic and mark centerhole so plastic will fit snug overthe core. The core opening canbe cut with a jig or coping sawor shaped by hand. If the lattermethod is used, drill a seriesof holes within the markedarea to remove waste (Fig.3A), then file to dress it to sizeand shape. One spool endshould be drilled and tapped sothe unit can be attached to thebase top. The other piece hasan opening for the start andfinish ends of the winding (Fig. 3B).

Winding Space is Provided by driving asteel pin in the small hole farthest from theend and then slipping on the spool end withthe four tapped holes to rest against the pin.Press the start-finish spool end on the corejust far enough in so the second pin willhold it.

Wrap a turn of 0.010 armature paperaround the core between the ends, takingcare to have the insulation come fully up tothe ends so the wire turns cannot touch themetal core. Hold the insulation together witha piece of cellophane tape. If armature paperis not available, substitute two turns of heavywrapping paper and give this a coat ofshellac.

The winding consists of 20 layers of #20heavy Formvar magnet wire laid with theturns close together, which with the windingspace provided will average about 40 turnsto a layer or about 800 total turns. Equipthe start and finish ends with a 3-in.-lengthof spaghetti tubing and carry them out theholes in the plastic spool end. The coil wireshould be put on neatly and tight to makea good job of it. If a winding machine orlathe is available, this would be the bestway to do it, but the wire can be puton by hand-winding with a bit ofpatience.

Fit the plastic tubing down over thecore (Fig. 4) and mark the position ofthe stop pins. Use a file to cut the twonotches (Fig. 3C) so the tube end willrest squarely on the spool.

Next Step is to make the coil en-closure (Fig. 5). Metal cannot be usedfor the base, because it surrounds thecore and current could be induced inthe metal to cause heating and also robthe coil of some of its energy neededfor the experiments. Use wood or some

Solder coil and line leads to switch before attaching it to the base witha washer and lockout.

other insulating material.Gum plywood worked out well for me and

it can be glued and bradded together. Cutthe pieces to dimension (Fig. 5) from 3/16-in.plywood. Make a 7/8-in.-diameter hole in thetop so the plastic tube that fits over the corewill rest on the spool end, then mark theposition of the screw openings from the plas-tic spool end on the top and drill the open-ings. Bore a 15/32-in. hole for the pushswitch. You can drill a 5/16-in. opening inthe side for the line cord and use a piece offibre or plastic tubing pressed in the hole asan insulating bushing (Fig. 5), or make a1/4-in. hole and allow the cord to enterthrough this if you prefer.

Countersink the brads and fill with PlasticWood. Use a fine sandpaper to smooth thesurface and round off the corners. Finishwith an attractive hammertone gray that'ssprayed on from an aerosal can. Two coatsof paint may be required for a good finish.

The Switch Must be Capable of handlingthe heavy current surge of the inductive load,or coil in the circuit. The Aero 3D05-5Pmomentary contact switch with a 12 amprating at 125 volts will meet specifications

OCTOBER, 1962 97

Magnetic field at top of core (left) is only strong enough to induce a very small voltage and the lamp filament ishardly lighted, but when placed down close to the bottom of the core (right) the maximum flux cuts through the

turns of the small coil and voltage increased to light the lamp to full candlepower.

and its physical size is comparatively small.Don't attempt to use other types of pushswitches with a low amp rating, because theircontacts will quickly burn away.

Solder the switch (Fig. 6) into the cir-cuit or in series with the line cord after at-taching the coil-core unit to the wood basewith four 4-40x 3/8-in. round or binder headmachine screws through the top. Splice be-tween one lead wire and coil lead can bemade with a solderless connector (Fig. 7).Thoroughly remove the insulation on theends of the coil leads with sandpaper beforemaking the connections. After wires are con-nected to the switch, use a washer and lock-nut to attach the switch to the top of theenclosure.

This completes the work in the main unit.

Connect an AC ammeter in series with oneside of the line with clip leads and press theswitch button. If you have made the coilcorrectly, the current should be around 6amps. A higher current indicates shortedturns or not enough turns for the size of coilspecified.

Make the Aluminum Ring from a piece of1-in. aluminum pipe used by electricians asconduit. If you can't find conduit this size,these rings can be purchased cut-to-size (seeMaterials List).

This ring is very important to the opera-tion of the coil. It must have a heavy-wallthickness to carry the 100 amps or so that isinduced in it by electromagnetic induction.In addition to being a good conductor of elec-tricity it must be lightweight. Actual size

98 SCIENCE and MECHANICS

of the 1-in. pipe measures about 1-5/16-in.outside diameter (od) and 1-1/16-in. (id)which gives a very desirable 1/8-in. wall thick-ness.

The ring should be 1-1/8 in. long and theends dressed square and smooth with a fileor preferably in a lathe. Smooth the outsideof the ring with a fine abrasive paper forgood appearance.

If the ring is held down close to the maincoil for a moment the current registered onthe ammeter will be about 7 to 8 amps. Thisrepresents some added primary current whicha transformer draws from the line when thereis a load on the secondary. It will also befound that the ring becomes warm when helddown due to the heavy current flowing in it.

A Portable Secondary Coil (Fig, 10) re-quired for the lamp experiment (Figs. 8 and9) is made by winding 78 turns of #24 heavyFormvar or e.namel magnet wire on a dowelor other suitable form which will give thecoil, when taped, an id of about 7/8 in. so itcan be pressed over the plastic core tubing.Bind the turns with three narrow bands oftape to hold it together. Two pieces of #20flexible insulated wire are then soldered andtaped to the ends of the coil. The other endsof the leads are connected to a miniaturescrew-base socket and a 6.3-volt screw-basepilot lamp put in the socket.

Other accessories can be made up as re-quired for other experiments which may bedeveloped by the teacher or student.

Tire Pump BtALL the "works" you need to make a heavy

duty Bunsen burner for those big scienceproject heating jobs is contained in an oldhand tire pump equipped with a l-1/4" cylin-der (Fig. 1).

Take apart the piston, ball valve in the base,and the barrel from the base. Cut the tubeonto two pieces, the threaded end 8 in. longand a 1-1/4" section. Lay out for air-intakeopenings and cut out as in Figs. 2 and 3. Nextcut the air regulator and bend in to form astop. Slide over barrel, making sure it turnsfreely.

If necessary, redrill the air outlet hole inthe pump base with a 1/8" drill. Cut a pieceof 1/8" copper tubing, file a slight taper onone end, coat end with gasket compound andforce into the hole in the pump base. Bend upthe tubing so the jet is centered in the tube,and cut off 1/4" above the top of the base.Plug-solder end of tube, and bore a hole witha No. 54 drill for an orifice. Remove tube, coattapered end with gasket compound and as-

semble. With natural gas or a mixture ofnatural and manufactured gas, it may benecessary to increase the size of the orifice.

—ROBERT MICALS

OCTOBER, 1962