Physics Part 51

282. The cylinder and coil form what is called an _armature_. The armature is mounted so as to be revolved between the poles of the "U"-shaped magnets by means of a handle. As the armature revolves, the lines of force from the magnets pa.s.s through the coil first in one direction and then in the other. This repeated change in the lines of force pa.s.sing through the coil produces an E.M.F. which may be felt by holding in the hands the two wires leading from the armature coil. On turning the armature _faster_ the current is felt _much stronger_, showing that the E.M.F. in the coil increases as the rate of cutting the magnetic lines of force by the coils increases.

[Ill.u.s.tration: FIG. 281.--A magneto.]

[Ill.u.s.tration: FIG. 282.--A shuttle armature.]

[Ill.u.s.tration: FIG. 283.--The induced current has a field which opposes the motion of the magnet. The heavy line represents the direction of the induced current.]

=299. Lenz's Law.=--While one is turning the armature of a magneto if the two wires leading from its coil are connected, forming what is called a "short circuit," the difficulty of turning the armature is at once increased. If now the circuit is broken, the armature turns as easily as at first. The increased difficulty in turning the armature is due to the _current_ produced in the coil. This current sets up a magnetic field of its own that opposes the field from the steel magnets.

This opposition makes it necessary for _work_ to be done to keep up the motion of the coil when a current is pa.s.sing through it. This fact is called _Lenz's Law_. It may be expressed as follows: _Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field that opposes the motion._ Lenz's Law follows from the principle of conservation of energy, that energy can be produced only from an expenditure of other energy. Now since an electric current possesses energy, such a current can be produced only by doing mechanical work or by expending some other form of energy. To ill.u.s.trate Lenz's Law, suppose that the north-seeking pole of a bar magnet be inserted in a closed coil of wire. (See Fig. 283.) The current induced in the coil has a direction such that its lines of force will pa.s.s within the coil so as to _oppose_ the field of the bar magnet, when the north pole of the magnet is inserted so as to point to the left. That is, the north pole of the helix is at the right. Applying the right-hand rule to the coil, its current will then be _counter clockwise_. On withdrawing the magnet, the current reverses, becoming _clockwise_ with its field pa.s.sing to the left within the coil.

A striking ill.u.s.tration of the opposition offered by the field of the induced current to that of the inducing field is afforded by taking a strong electromagnet (see Fig. 284) and suspending a sheet of copper so as to swing freely between the poles. When no current flows through the magnet the sheet swings easily for some time. When, however, the coils are magnetized, the copper sheet has induced within it, currents that set up magnetic fields strongly opposing the motion, the swinging being stopped almost instantly. The principle is applied in good ammeters and voltmeters to prevent the swinging of the needle when deflected. The current induced in the metal form on which is wound the galvanometer coil is sufficient to make the needle practically "dead beat."

[Ill.u.s.tration: FIG. 284.--The magnetic field stops the swinging of the sheet of copper.]

=300. The Magneto and the Dynamo.=--Magnetos are used to _develop small_ currents, such as are used for telephone signals, and for operating the _sparking_ devices of gasoline _engines_. They are therefore found in automobiles containing gasoline motors. The most important device for producing electric currents by electromagnetic induction, however, is the _dynamo_. It is employed whenever large currents are desired. The principle of this device is similar to that of the magneto except that it contains an _electromagnet_ for producing the magnetic field.

Since the electromagnet can develop a much stronger field than a permanent magnet, the dynamo can produce a higher E.M.F. and a much larger current than the magneto.

[Ill.u.s.tration: LORD KELVIN

"By Permission of the Berlin Photographic Co., New York."

Lord Kelvin (Sir William Thomson), (1824-1907). Professor of Physics, Glasgow University. Invented the absolute scale of temperature: also many practical electrical measuring instruments. The foremost physicist of the latter part of the nineteenth century.]

[Ill.u.s.tration: MICHAEL FARADAY

"By Permission of the Berlin Photographic Co., New York."

Michael Faraday (1791-1867). Famous English Physicist. Made many discoveries in electricity and magnetism; "Greatest experimentalist of the nineteenth century."]

=301. The Magnetic Fields of Generators.=--In the magneto, the magnetic field is produced by _permanent_ steel magnets. In dynamos powerful _electromagnets_ are used. The latter are sometimes excited by currents from some other source, but usually current from the armature is sent around the field coils to produce the magnetic fields. Dynamos are cla.s.sified according to the manner in which the current is sent to their field coils.

[Ill.u.s.tration: FIG. 285.--A series-wound dynamo.]

[Ill.u.s.tration: FIG. 286.--A shunt-wound dynamo.]

[Ill.u.s.tration: FIG. 287.--A compound-wound dynamo.]

_A._ The _series wound dynamo_ (see Fig. 285) is arranged so that _all_ of the current produced by the armature is sent through coils of coa.r.s.e wire upon the fields, after flowing through the external circuit.

_B._ The _shunt wound dynamo_ (see Fig. 286) sends a part only of the current produced through the field coils. The latter are of many turns of fine wire so as to use as little current as possible. The greater part of the current goes to the main circuit. If the number of lamps or motors connected to the main circuit is increased, the voltage is lessened which weakens the current in the field coils, causing a weaker field and still lower voltage, producing a fluctuating E.M.F. which is unsatisfactory for many purposes. This fault is overcome by

_C._ the _compound wound dynamo_. This dynamo has both shunt and series coils upon its fields. (See Fig. 287.) If more current is drawn into the main circuit with this dynamo, the series coils produce a stronger field compensating for the weaker field of the shunt coils, so that uniform voltage is maintained. The compound wound generator is therefore the one most commonly employed.

Important Topics

1. Laws of electromagnetic induction (a) conditions, (b) E.M.F., (c) direction.

2. Devices, (a) magneto, (b) dynamo: series, shunt, compound.

3. Ill.u.s.trations of the laws.

Exercises

1. Under what conditions may an electric current be produced by a magnet?

2. Show how Lenz's Law, follows from the principle of conservation of energy.

3. A bar magnet is fixed upright with its north-seeking pole upward. A coil is thrust down over the magnet. What is the direction of the current induced in the coil? Explain.

4. In what two ways may a current be induced in a closed coil?

5. What method is employed in the magneto? In the dynamo?

6. What is the nature of the current produced in the armature coil of a magneto, that is, is it direct or alternating? Why?

7. What is the resistance of a 20-watt tungsten lamp if the E.M.F. is 115 volts?

8. Find the resistance of a 40-watt tungsten lamp when the voltage is 115? How much heat will it produce per minute?

9. An Edison storage battery cell on a test gave a discharge of 30 amperes. The average voltage was 1.19. What was the resistance of the cell?

10. Eight storage cells are connected in series. Each has an E.M.F. of 1.2 volts and an internal resistance of 0.03 ohms. What will be the current flowing through a voltmeter having 500 ohms resistance in circuit with them?

(2) THE DYNAMO AND THE MOTOR

=302. The Dynamo= may be defined as a machine for transforming mechanical energy into the energy of electric currents by electromagnetic induction. Although electromagnetic induction was discovered in 1821, practical dynamos were not built for about 40 years or until between 1860 and 1870. The great development in the production and use of electric currents has come since the latter date. The principle parts of the dynamo are (a) the _field magnet_, (b) the _armature_, (c) the _commutator_ or _collecting rings_, (d) the _brushes_. Fig. 288 shows several common methods of arranging the field coils and the armature.

[Ill.u.s.tration: FIG. 288.--Several methods of arranging the field coils and the armature of a dynamo.]

[Ill.u.s.tration: FIG. 289.--A drum armature.]

The field coils vary in number and position. The purpose of their construction is always to send the largest possible number of lines of force through the armature. Some dynamos are _bipolar_, or have _two_ poles, others are multipolar or have more than two. In Fig. 288 No. 4 has four poles. The _armature_ of a dynamo differs from a magneto armature in that it consists of a series of coils of insulated copper wire wound in numerous slots cut in the surface of a cylindrical piece of iron. Fig. 289 shows a _side_ view of the iron core of such an armature. Iron is used to form the body of the armature since the magnetic lines of force flow easily through the iron. The iron by its permeability also concentrates and increases the magnetic flux. The best armatures are made of many thin sheets of soft iron. These are called _laminated_ armatures. An armature made of a solid piece of iron becomes hot when revolving in a magnetic field. This is due to electric currents induced in the iron itself. This heating is largely reduced by _laminating_ the armature. Why?

[Ill.u.s.tration: FIG. 290.--Armature connected to slip rings producing an alternating current.]

=303. Methods of Collecting Current from the Armature.=--The electric currents produced in the armature are conducted away by _special sliding contacts_. The stationary part of the sliding contact is called a _brush_. The moving part is a _slip ring_ or a _commutator_. Fig. 290 shows an armature coil connected to slip rings. As the armature revolves, the coils and slip rings revolve with it. The two ends of the armature coils are connected to the two rings respectively. Now as the armature revolves it cuts the lines of force first in one direction and then in the other. This produces in the coils an E.M.F. first one way and then the other. This E.M.F. sets up a current which is conducted to the outside circuits through the slip rings and brushes. Such a current which repeatedly reverses its direction is called an _alternating current_. Fig. 291 (1) indicates graphically how the current moves alternately one way and then the other. Alternating currents are extensively used for electric _light, heat, and power_. _Direct currents_ or those going continuously in one direction are however in much demand especially for _street car service_, _for electrolysis_, and for _charging storage batteries_.

=304. The Commutator.=--For a dynamo to deliver a _direct current_ it must carry upon the shaft of the armature a _commutator_. The commutator is used to _reverse_ the connections of the ends of the armature coils at the instant that the current changes its direction in the armature.

This reversal of connection when the direction of current changes, keeps the current in the outside circuit flowing in the same direction. Fig.

291 is a diagram of an armature with a commutator. The commutator is a _split ring_, having as many parts or _segments_ as there are coils upon the armature. The brushes touch opposite points upon the commutator as they slide over the surface of the latter. Suppose that the armature viewed from the commutator end rotates in a counter-clockwise direction, also that the currents from the upper part move toward the commutator and out the top brush.

[Ill.u.s.tration: FIG. 291.--The armature coils are connected to a commutator producing a direct current.]

As the armature revolves, its coils soon begin to cut the force lines in the opposite direction. This change in the direction of cutting the lines of force causes the current to reverse in the coils of the armature. At the instant the current changes in direction, what was the upper segment of the commutator slips over into contact with the lower brush, and the other segment swings over to touch the upper brush. Since the current has reversed in the coils it continues to flow out of the upper brush. This change in connection at the brushes takes place at each half turn of the armature, just as the current changes in direction in the coils. This is the manner in which the commutator of a dynamo changes the alternating current produced in the armature coils, into a direct current in the external circuit. Fig. 292 (1) represents graphically an alternating current, (2) of the same figure shows current taken from the brushes of the commutator of a dynamo with one coil on the armature.

[Ill.u.s.tration: FIG. 292.--Graphic representation of (1) an alternating current; (2) a pulsating current; (3) a continuous current.]