Chapter XXV: Induced Currents

_=426. Electromagnetic Induction.=_ You have seen, by experiments, that a magnet has the power to induce another piece of iron or steel to become a magnet. You have also seen, in the study of static electricity, that an electrified body has the power to act through space upon another conductor. A body may be polarized and charged with static electricity by induction.

Several questions now come up. Can a _current_ of electricity in a conductor induce a _current_ in another conductor not in any way connected with the first? Can current electricity produce effects through space? Is there an electromagnetic induction?

It has been seen that a current-carrying wire has a magnetic field, and that magnetic fields can act through space. It is evident, then, that a conductor will be surrounded and cut by lines of force when it is placed in a magnetic field, or near a wire or coil through which a current passes. Let us study this by experiments.

=EXPERIMENTS 175-182. To study induced currents.=

_Apparatus._ The two coils of wire (Nos. 89, 90); two short,
soft iron cores (Nos. 92, 93); long iron core (No. 96); bar
magnet (No. 97); astatic galvanoscope (No. 59); dry cell (No.
51); key (No. 55); horseshoe magnet; connecting wires with
spring connectors (No. 54) on the ends (§ 226-230); coil of
wire (No. 98) wound on an iron core; compass.

=EXPERIMENT 175. To find whether a current can be generated
with a bar magnet and a hollowed coil of wire.=

=427. Directions.= (A) Arrange as in Fig. 136. The coil (No.
90) of fine wire is joined to A G (No. 59) as shown. Small
pieces of tin or copper, 1 and 2, are used to make connections
between the coil ends and wires, 3 and 4, which are attached
to the galvanoscope. It is best to use the wires, 3 and 4, so
that the coil will be 2 feet at least, from A G; otherwise the
needle of A G might be affected by the magnet, M (No. 97).

(B) Get clearly in mind in which direction the right-hand end
of the needle is deflected when a current enters A G at L, the
left-hand binding-post. If you have forgotten the results of
previous experiments, use the cell for an instant, touching the
wire from the carbon to L and that from the zinc to R. If any
currents come from the coil, later, you should be able to tell
in which direction they flow, the coil and A G forming a closed
circuit.

(C) Hold the magnet, M, as shown, and quickly push it into
the coil until it has the place of a core, at the same time
watching the needle. If a current is produced, in which
direction does it flow from the coil? Does the needle remain
deflected? Is the current constant or temporary?

(D) After the magnet, M, has been placed in the coil, as in
(C), and the needle has come to rest, quickly pull M from the
coil, watching the needle. If a current is produced, does it
pass from the coil in the same direction as before, in (C)?

(E) Turn M end for end, repeat (C) and (D), and study the
results. Are lines of force made to cut the turns of the coil?

(F) Repeat (C) and (D), moving M slowly.

_=428. Discussion.=_ An induced current, produced as in the above experiment, is a momentary one. No current passes when the magnet and coil are still; at least one of them has to be in motion. When the magnet is inserted, the induced current is said to be an _inverse_ one, as it passes in a direction opposite to that which would be necessary to give the magnet its poles, it being considered a core magnetized by the current. A _direct_ current is produced when the magnet is withdrawn from the coil. Rapid movements produce stronger currents than slow ones. (See § 439.)

_=429. Induced Currents and Work.=_ It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. When the coil is pushed on to the magnet, or when it is moved through a magnetic field, force is also required. We have, in this simple experiment, the key to the action of the dynamo and other important electrical machines. These will be discussed later.

=EXPERIMENT 176. To find whether a current can be generated
with a bar magnet and a coil of wire having an iron core.=

=430. Directions.= (A) Arrange as in Exp. 175, Fig. 136, and,
in addition, place an iron core (No. 92) inside of the coil
(No. 90).

(B) Hold the bar magnet (No. 97) as in Fig. 136, and quickly
lower it until it touches the core, at the same time watching
the needle. Study results, direction of current, etc., as
before.

(C) Suddenly withdraw M from the core. Is the current produced
in the same direction as that from (B)?

(D) Turn M end for end and repeat (B) and (C).

(E) Repeat (C) and (D), moving magnet slowly.

How does the strength of the current compare with that of Exp.
175? Are lines of force made to cut the turns of the coil?

=EXPERIMENT 177. To find whether a current can be generated
with a horseshoe magnet and a coil of wire having an iron core.=

=431. Directions.= (A) Arrange the apparatus as in Exp. 176,
but use the horseshoe magnet, H M, instead of the bar magnet.
Fig. 137 shows the coil (No. 90) with one pole of H M held over
the core.

(B) Study the effect of quickly lowering and raising first one
pole and then the other over the core, as with the bar magnet.
Get clearly in mind the direction in which the induced current
flows in each case.

_=432. Induced Currents and Lines of Force.=_ In the experiments just given, it should be remembered that the permanent magnets are sending out thousands of lines of force from their N poles, and receiving them again at their S poles. As the magnet is pushed into the coil (Exp. 175), the lines of force not only cut through the turns of the coil, but the number of lines of force that cut the coil at any instant varies rapidly as the magnet is moved.

Motion is necessary, with this arrangement, to make a change in the number of cutting lines of force. The current passes only while the magnet moves; and the direction of the current at any moment depends upon whether the number of lines of force is increasing or decreasing at that moment. (See § 438, 439.)

=EXPERIMENT 178. To find whether a current can be generated
with an electromagnet and a hollow coil of wire.=

=433. Directions.= (A) The hollow coil (No. 90) should be
joined to the astatic galvanoscope, as shown in Fig. 136.
Instead of the bar magnet in Fig. 136, an electromagnet is to
be used, and this should be joined in series with a cell and
key, as shown in Fig. 138. The current from the cell will pass
only when K is pressed.

(B) Note from the winding which way the current must pass
around the coil when the circuit is closed at K, and determine
whether the lower end of the long iron core, L I C (No. 96)
should be N or S. With the compass test the poles of the core
to be sure you are right.

(C) Quickly lower the end of L I C into the hollow coil (H,
Fig. 136), the circuit being kept closed long enough to allow
the needle to partially come to rest again. Withdraw L I C
before you open the circuit. Explain action of needle.

(D) Reverse the direction of the current through the
electromagnet, by changing the connections, and repeat (C).
Does any induced current pass through A G when the core is held
still in the coil H, even though a current passes through coil
E?

=EXPERIMENT 179. To find whether a current can be generated
with an electromagnet and a coil of wire having an iron core.=

=434. Directions.= (A) Fig. 139 shows simply the arrangement of
coils. Coil H (No. 90) with core, is joined to the galvanoscope
as in Fig. 136. Coil E, with short core, should be joined to
key and cell as shown in Fig. 138.

(B) Keeping in mind the polarity of the lower end of core E,
quickly lower it to the core of H, the circuit being kept
closed for a few seconds. Does the needle remain deflected
after the motion ceases?

(C) Quickly raise E, the circuit being still closed, then
open the circuit. Compare the directions taken by the induced
currents in (B) and (C).

_=435. Discussion of Exps. 178, 179.=_ This motion in straight lines is not suitable for producing currents strong enough for commercial purposes. In order to produce currents of considerable strength, the coils of wire have to be pushed past magnets with great speed. Special machines (see Dynamos) are constructed in which the coils are wound so that they can be given a rapid _rotary motion_ as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them.

=EXPERIMENT 180. To study the effect of starting or stopping a
current near a coil of wire or other closed circuit.=

=436. Directions.= (A) Arrange as in Fig. 140. Place the two
coils, H and E, on the same core, L I C. Connect E with the
key and cell as before (Fig. 138). Connect H with the astatic
galvanoscope, A G, as in Fig. 136. Keep the coils 2 or 3 feet
from A G, so that the needle will not be affected by them.

(B) Close the circuit at the key, watching the needle, then
as soon as the needle regains its former position, open the
circuit again. Compare the direction of the induced current in
H with that of the current in E, (1) when the main circuit is
closed, and (2) when it is opened. Is any current induced in H
by a steady current in E? (See Transformers.)

=EXPERIMENT 181. To study the effect of starting or stopping a
current in a coil placed inside of another coil.=

=437. Directions.= (A) Arrange as in Fig. 141. Join coil H with
the astatic galvanoscope, A G. Place the small coil P (No. 98)
with core, inside of H, and connect the ends of P with the key
and cell, as shown.

(B) Close the circuit at K; watch the needle, and as soon as it
regains its position, open the circuit again.

Compare the direction of the induced current in H with that of
the inducing current in P, (1) when the inducing circuit is
closed, and (2) when it is broken. (See Induction Coils.)

_=438. Discussion of Exps. 180, 181.=_ When a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced.

We may consider that when the inducing circuit is closed, the lines of force shoot out through the turns of the outside coil. Upon opening the circuit the lines of force cease to exist; that is, we may imagine them drawn in again.

_=439. Direction of Induced Current.=_ Fig. 142 shows the magnet on its way into the coil; the number of lines of force is increasing in the coil, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an _indirect_ current. If a current from a cell were passed through the coil in the direction of this indirect current, the lower end of a bar of iron would become a S pole. (See § 428.)

_=440. Laws of Induction.=_ (1) An increase in the number of lines of force that pass through a closed circuit produces an indirect induced current; while a decrease produces a direct one. (See § 428.)

(2) The E. M. F. of the induced current is equal to the rate of increase or decrease in the number of lines of force that pass through the circuit.

(3) A constant current produces no induced current, provided there is no motion.

(4) Closing a circuit produces an indirect current.

(5) Opening a circuit produces a direct current.

(6) _Lenz's Law._ Induced currents have a direction that tends to stop the motion that produces them.

_=441. Primary and Secondary Currents.=_ In the preceding experiments in induction, it must be kept in mind that the current from the cell did not pass through the galvanoscope. There were two entirely separate circuits, in no way connected. The _primary_ current comes from the cell, while the _secondary_ current is an induced one.

=EXPERIMENT 182. To see what is meant by alternating currents.=

=442. Directions.= (A) Arrange as in Fig. 143. Connect coil H
with A G, as before. Place one pole of H M against the end of
the core I C, hold H with one hand, and with the other quickly
push the other pole of H M onto the core. This should produce a
momentary current through A G, first in one direction, and then
in the other. Let the needle come to rest.

(B) Move H M back and forth upon the end of I C, changing
its polarity rapidly. A minute's practice will enable you to
slide the core from one pole of H M to the other and back
again rapidly--3 complete vibrations per second being about
right. The needle should be parallel to the coil of A G, and
if properly done, the needle will be made to vibrate back and
forth slightly at each change in the polarity of I C.

_=443. Direct and Alternating Currents.=_ A current that flows steadily in one direction is said to be a _direct_ current. A cell gives a direct current when the circuit is closed. When the current passes in one direction for an instant, and then reverses immediately and flows in the opposite direction, it is said to _alternate_. The induced current which flowed through the galvanoscope in Exp. 182 was an alternating one. Currents of this class have great practical uses.

_=444. Self-Induction; Extra Currents.=_ It has been shown that a magnetized coil can act through space and induce a current in a neighboring coil. The lines of force which reach out from an electromagnet will generate a current in any conductor which happens to be in the field, or which is moved across the lines. It is evident, then, since the lines of force from each turn of a coil cut all the other turns of the same coil, that each turn acts as a conductor placed in the field of every other turn. The instant a current begins to flow through a coil, there is an inverse current of self-induction started in the coil, which opposes the current in the cell. When the circuit is broken, this _extra current_, as it is also called, is a direct one and adds its strength to that of the current from the cell; as this takes place at the instant the circuit is broken, a bright spark is seen at the key, and this shows that the E. M. F. of this extra current is high. Practical uses are made of it.