Physics Part 44

(1) THE MAGNETIC EFFECT OF ELECTRIC CURRENTS

=255. The Magnetic Effect.=--Of all the effects of electric currents, it is generally conceded that the _magnetic effect_ is the one of _greatest practical importance_, and it is also the one most extensively used. An experiment ill.u.s.trating this effect has been described in Art. 239. This experiment shows that an electric current, if _parallel_ to a magnetic needle, and _near it_ will deflect the north-seeking pole of the needle to the right or left depending upon the _direction_ of the current flow.

This _deflection_ of the magnetic needle is due to the fact that surrounding every electric current are magnetic lines of force. It is this magnetic field of the current that causes the needle to turn. The position taken by the needle is the resultant of the forces of two magnetic fields; one, the earth's field, the other, that of the current.

=256. Right-hand Rule for a Conductor.=--To show the presence of the magnetic field about a current, pa.s.s a thick copper wire vertically through a sheet of paper, and connect the ends of the wire to a source of current. While the current (this should be as much as 10 amperes if possible) is flowing, sprinkle iron filings upon the paper and tap gently. The filings will arrange themselves in circles about the wire showing the magnetic field. (See Fig. 229.) The needle of a magnetoscope tends to place itself parallel to the lines of force of this field and from this action or tendency the _direction_ of the magnetic lines about a current may be determined. The following rule is helpful and should be memorized: _Grasp the conductor with the right hand with the outstretched thumb in the direction that the current is flowing. The fingers will then encircle the wire in the direction of the lines of force._ This rule may be reversed, for, if the fingers of the right hand grasp the wire so as to point with the magnetic field, then the current flows in the direction in which the thumb points. (See Fig. 230.)

[Ill.u.s.tration: FIG. 229.--Magnetic field about a wire carrying an electric current.]

[Ill.u.s.tration: FIG. 230.--Right-hand rule for the magnetic field of a current.]

=257. Magnetic Field of a Helix.=--If a wire be wound about a cylinder to form a cylindrical coil with parallel turns, it forms a _helix_ or _solenoid_. The shape of the magnetic field about a current depends upon the _form_ of the conductor. If the latter is in the form of a _helix_ its magnetic field resembles that of a straight bar magnet. (See Fig.

231). In fact the helix _has the properties of a magnet_ with north- and south-seeking poles while a _current_ is _flowing_ through it. If such a coil is suspended so as to turn freely, it tends to turn until the field within it is parallel to the earth's magnetic field. Such a suspended helix may therefore be used as a compa.s.s. In order to strengthen the magnetic field of a helix or solenoid, the s.p.a.ce within its turns is filled with iron, often in the form of small soft-iron wires. This bundle of iron wire is called the _core_ of the helix. The core becomes strongly magnetized by the field of the helix while the current is flowing and quickly loses its magnetic force when the current is stopped. _The direction of the current in a helix_ (Fig. 232) or the _polarity_ of its core may be determined by another _right-hand rule_.

_If the helix is grasped with the right hand so that the fingers point in the direction in which the current is flowing, the extended thumb will point in the direction of the north pole of the helix._ On the other hand, if the poles of the helix are known, then, when the helix is grasped with the right hand so that the thumb points to the north-seeking pole, the current is flowing in the wires in the direction that the fingers point.

[Ill.u.s.tration: FIG. 231.--The magnetic field of a helix.]

[Ill.u.s.tration: FIG. 232.--Right-hand rule for a helix.]

=258. The Electromagnet.=--These "right-hand" rules are applied in many different devices. Among these, perhaps the most important is the electromagnet, which is used in the electric bell, the telegraph, the telephone, the dynamo, the motor, and many other electric contrivances.

The electromagnet is defined as a _ma.s.s of iron around which is placed a helix for conducting an electric current_. On account of its large permeability, the iron core of the helix adds greatly to the effectiveness of the electromagnet, since the magnetism of the iron is added to that of the current in the helix. The magnetism remaining in the iron after the current stops is called the _residual_ magnetism. The residual magnetism is small when the core is made of small wires or thin plates, but is larger when the iron core is solid. Like artificial steel magnets, electromagnets are usually of two forms, _bar_ and _horseshoe_.

(See Figs. 233 and 234.) For most purposes the horseshoe form is the more effective since it permits a complete iron circuit for the magnetic lines of force. (See Fig. 235.) This is the form used in the electric bell, in the telegraph sounder, and in lifting magnets. (See Fig. 236.)

[Ill.u.s.tration: FIG. 233.--A bar electromagnet.]

[Ill.u.s.tration: FIG. 234.--A horseshoe electromagnet.]

[Ill.u.s.tration: FIG. 235.--A horseshoe electromagnet may have a complete iron circuit for its lines of force.]

[Ill.u.s.tration: FIG. 236.--A lifting magnet.]

=259. Effective Electromagnets.=--The _magnetic_ effect of a current in a helix is small, hence the force usually is increased by inserting a core of iron. When at first man tried to signal with electromagnets at a distance it was found that the current would not work the electromagnet. An American by the name of Joseph Henry discovered the remedy for this condition. He found that if the copper wire was insulated by wrapping silk thread about it, and then many layers of the silk insulated wire were wound upon a spool with an iron core, that the magnet would work at a great distance from the source of current. If the current is increased, the magnet is stronger than at first. Thus an _electromagnet may be made stronger by_ (a) _increasing the number of turns of wire in its coils_ and by (b) _sending a stronger current through it_.

[Ill.u.s.tration: FIG. 237.--A simple telegraph circuit.]

=260. The Telegraph.=--The invention of an effective electromagnet by Henry made possible the _electric telegraph_. In its simplest form it consists of a battery, _C_, a key, _K_, and a sounder, _S_, with connecting wires. (See Fig. 237.) The _sounder_ (Fig. 238) contains a _horseshoe electromagnet_ and a bar of soft iron across its poles called an armature, _A_, attached to a lever _L_. When the key is closed, the electromagnet draws down the armature and lever until the latter hits a stop _O_, making a click. When the key is raised, the magnet releases the armature which is raised by the action of a spring at _S_ until the lever hits a stop at _T_ making another click. Closing and opening the circuit at _K_ will start and stop the current which operates _S_ which may be 100 miles or more from _K_. One voltaic cell will work a sounder in the same room. But if many miles of wire are in the circuit, the E.M.F. of a single cell will not force sufficient current through the long wire to operate the sounder.

[Ill.u.s.tration: FIG. 238.--A telegraph sounder.]

[Ill.u.s.tration: FIG. 239.--A telegraph relay.]

[Ill.u.s.tration: FIG. 240.--How the relay is used.]

[Ill.u.s.tration: Samuel F. B. Morse (1791-1872). Inventor of the electromagnetic recording telegraph and of the dot and dash alphabet.

SAMUEL F. B. MORSE

"From Appleton's Cyclopedia of American Biography, Copyright 1888 by D.

Appleton & Co."]

[Ill.u.s.tration: Thomas A. Edison, Orange, New Jersey. Invented the incandescent lamp; phonograph; moving picture; most noted inventor of electrical appliances of the present day.

THOMAS A. EDISON

"Copyright, Photographische Gessellschaft," and "By Permission of the Berlin Photographic Co., New York."]

A battery of several cells is then required. Even a large battery is insufficient to operate a long line containing many sounders in circuit.

Recourse is therefore usually made to a more sensitive device called a _relay_. (See Fig. 239.) In the relay a very small current will magnetize its electromagnet enough to draw toward it the delicately hung armature thereby closing a second circuit which contains a sounder and a battery. (See Fig. 240.) when the current in the main circuit is stopped, the armature of the relay is drawn back by a light spring. This opens the _local_ circuit. Thus the local circuit is closed and opened by the relay just in time with the starting and stopping of the current in the main line. It is thus possible for a small current in the main line by the use of a relay, to close and open a second local circuit containing a local battery and sounder. Modern telegraph lines are operated in this manner.

[Ill.u.s.tration: FIG. 241.--An electric bell and its circuit.]

=261. The electric bell= (see Fig. 241), consists of an electromagnet, _M_, a soft iron _armature_, _A_, attached to the _tapper_, _T_, and a post, _R_. When no current is flowing a spring at _S_ holds the armature against the post _R_. When current flows through the helix, its core becomes magnetized and attracts the armature, drawing it away from the post, _R_, and causing the tapper to hit the bell. Drawing _A_ away from the post, however, breaks the circuit at _R_ and the current stops. The magnetism in the core disappears releasing the armature, which is then pulled back by the spring _S_ against the post _R_. This completes the circuit and the process repeats itself several times a second as long as the current flows.

[Ill.u.s.tration: FIG. 242.--Magnetizing by the discharge of a Leyden jar.]

=262. Static and Current Electricity Compared.=--The likeness between a discharge of static electricity and an electric current may be shown by winding a coil of insulated wire about a gla.s.s tube which contains a steel needle. If a Leyden jar (see Fig. 242) is discharged through the coil the steel needle is usually found to be magnetized, showing that the discharge of the static electricity has a magnetic effect similar to that of an electric current. Sometimes a given end of the needle has a north pole and at other times a south pole. This is believed to indicate that the charge of the Leyden jar is _oscillatory_, and that in different discharges sometimes a surge in one direction and at other times a surge in the reverse direction has been most effective in magnetizing the needle. Compare this action with that described in Art.

233.

Important Topics

1. Right-hand rules, for conductor, for helix.

2. The electromagnet, two forms, where used?

3. Likeness between static and current electricity.

4. The electric bell, parts, action.

5. The telegraph, key, sounder, relay.

Exercises

1. What is the difference between an electric charge and a current?

2. How can a magnetic effect be produced from an electric charge?

3. What is a magnetic field? Give two evidences of a magnetic field about a current in a wire?

4. A current is flowing north in trolley wire, what is the direction of the magnetic field under the wire? Explain.

5. What would be the result if a hard steel core were placed in the electromagnet? Explain.

6. If the north-seeking pole of a helix is facing you, does the current in the coils before you move in a clockwise or in a counter-clockwise direction? Explain.

7. A helix is placed horizontally with its north-seeking pole toward the north. Does the current in the wire at the top of the helix move east or west? Explain.