meter coil and the series resistance is 100 мa 100 22 R=] - = 0.0001 - = 10,000 ohms, M 1 VOLT 100 VOLT and the series resistance alone is
• -.01 V -- 1000 VOLT R, = 10,000 - 100 = 9,900 ohms.
9,900 g 25 006 666 wn 9,999,900 R ment with the required resistances connected in Multirange voltmeters utilize one meter moveseries with the meter by a convenient switching arrangement. A multirange voltmeter circuit with three ranges is shown in figure 8-134. The total circuit resistance for each of the three ranges beginning with the 1-volt range is:
1 R = 7 = 100 = 0.01 megohm FIGURE 8-134. Multirange voltmeter schematic. 100 100 = 1 megohm to be measured. Because the current through the meter circuit is directly proportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed series resistance.
For example, assume that the basic meter (microammeter) is to be made into a voltmeter with a full-scale reading of 1 volt. The coil resistance of the basic meter is 100 ohms, and
0.0001 ampere (100 microamperes) causes a fullscale deflection. The total resistance, R, of the VOLTS -I5V
0-1.5V- — O-150V FIGURE 8-135. Typical multirange voltmeter.
1,000 100 = 10 megohms.
Multirange voltmeters, like multirange ammeters, are used frequently. They are physically very similar to ammeters, and their multipliers are usually located inside the meter with suitable switches or sets of terminals on the outside of the meter for selecting ranges (see figure 8-135).
Voltage-measuring instruments are connected across (in parallel with) a circuit. If the approximate value of the voltage to be measured is not known, it is best, as in using the ammeter, to start with the highest range of the voltmeter and progressively lower the range until a suitable reading is obtained.
In many cases, the voltmeter is not a centralzero indicating instrument. Thus, it is necessary to observe the proper polarity when connecting the instrument to the circuit, as is the case when connecting the d.c. ammeter. The positive terminal of the voltmeter is always connected to the positive terminal of the source, and the negative terminal to the negative terminal of the source, when the source voltage is being measured. In any case, the voltmeter is connected so that electrons will flow into the negative terminal and out of the positive terminal of the meter. In figure 8-136 a multimeter is properly connected to a circuit to measure the voltage drop across a resistor. The function switch is set at the d.c. volts position and the range switch is placed in the 50-volt position.
Red DCV Black 0-30V
VARIABLE SOURCE FiGURE 8-136. A multimeter connected to measure a circuit voltage drop.
The function of a voltmeter is to indicate the potential difference between two points in a circuit. When the voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has low resistance, it will draw an appreciable amount of current. The effective resistance of the circuit will be lowered, and the voltage reading will consequently be lowered.
When voltage measurements are made in high-resistance circuits, it is necessary to use a high-resistance voltmeter to prevent the shunting action of the meter. The effect is less noticeable in low-resistance circuits because the shunting effect by increasing the strength of the permanent magnet, by using lighter weight materials for the moving element (consistent with increased number of turns on the coil), and by using sapphire jewel bearings to support the moving coil.
Voltmeter Accuracy The accuracy of a meter is generally expressed in percent. For example, a meter with an accuracy of 1 percent will indicate a value within 1 percent of the correct value. The statement means that, if the correct value is 100 units, the meter indication may be anywhere within the range of 99 to 101 Voltmeter Sensitivity OHMMETERS The sensitivity of a voltmeter is given in ohms Two instruments are commonly used to check per volt (Q/E) and is determined by dividing the the continuity or to measure the resistance of a resistance (Rm) of the meter plus the series circuit or circuit element. These instruments are resistance (Rs) by the full-scale reading in volts. the ohmmeter and the megger, or megohmmeter.
The ohmmeter is widely used to measure resistance sensitivity = Rm + R. and to check the continuity of electrical circuits and devices. Its range usually extends to a few E inegohms. The megger is widely used for measuring This is the same as saying that the sensitivity is insulation resistance, such as the resistance equal to the reciprocal of the current (in amperes); between the windings and the frame of electric that is, machinery, and for measuring the insulation resistance of cables, insulators, and bushings. Its sensitivity = ohms 1 range may extend to more than 1,000 megohms.
volts volts amperes When measuring very high resistances of this ohms nature, it is not necessary to find the exact value of resistance, but rather to know that the insula- Thus, the sensitivity of a 100-microampere tion is either above or below a certain standard.
movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
The sensitivity of a voltmeter can be increased When precision measurements are required, some type of bridge circuit is used. Ohmmeters may be of the series or shunt type.
OHMS E. _ 4.5V 37.5K 13.5K 9K 4.5K ISK B 0-1 ma 2.5
MILLIAMPERES FIGURE 8-137. Ohmmeter circuit.
Series-type Ohmmeters A simplified schematic of an ohmmeter is shown in figure 8-137. E is a source of EMF; R, is a variable resistor used to zero the meter; R2 is a fixed resistor used to limit the current in the meter movement; and A and B are test terminals across which the resistance to be measured is placed.
If A and B are connected together (shortcircuited), the meter, the battery, and resistors R1 and R2 form a simple series circuit. With R1 adjusted so that the total resistance in the circuit is 4,500 ohms, the current through the meter is 1 ma. and the needle deflects full scale. Since there is no resistance between A and B, this position of the needle is labeled zero (figure 8-138). If a resistance equal to 4,500 ohms is placed between terminals A and B, the total resistance is 9,000 ohms and the current is .5 ma.
This causes the needle to deflect half scale. This half-scale reading, labeled 4.5 K ohms, is equal to the internal resistance of the meter, in this instance 4,500 ohms. If a resistance of 9,000 ohms is placed between terminals A and B, the needle deflects one-third scale. Resistances of 13.5 K and
1.5 K placed between terminals A and B will cause a deflection of one-fourth and three-fourths scale, respectively.
If terminals A and B are not connected (opencircuited), no current flows and the needle does not move. The left side of the scale is, therefore, labeled infinity to indicate an infinite resistance.
A typical ohmmeter scale is shown in figure
8-138. Note that the scale is not linear and is crowded at the high resistance end. For this reason, it is good practice to use an ohmmeter range in which the readings are not too far from mid-scale. A good rule is to use a range in which the reading obtained does not exceed ten times, or is not less than one-tenth, the mid-scale reading.
FIGURE 8-138. A typical ohmmeter scale.
The useful range of the scale shown is, by this rule, from 450 ohms to 45,000 ohms.
Most ohmmeters have more than one scale.
Additional scales are made possible by using various values of limiting resistors and battery voltages. Some ohmmeters have a special scale called a low-ohm scale for reading low resistances.
A shunt-type ohmmeter circuit is used for this scale.
Shunt-Type Ohmmeter Shunt-type ohmmeters are used to measure small values of resistance. In the circuit shown in figure 8-139, E (voltage) is applied across a limiting resistor R and a meter movement in series.
Resistance and battery values are chosen so that the meter movement deflects full scale when terminals A and B are open. When the terminals are short-circuited, the meter reads zero; the short circuit conducts all the current around the meter.
The unknown resistance Rz is placed between terminals A and B in parallel with the meter movement. The smaller the resistance value being measured, the less current flows through the meter The value of the limiting resistor R is usually made large compared to the resistance of the meter movement. This keeps the current drawn from the battery practically constant. Thus, the value of R.
determines how much of this constant current flows through the meter and how much through Rza Note that in a shunt-type ohmmeter, current is always flowing from the battery through the meter 330
(2) Short the leads together and set the meter to read zero ohms by setting the zero adjustment. If the scale is changed, readjust to zero ohms.
(3) Connect the unknown resistance between the test leads and read its resistance from the scale. Never attempt to measure resistance in a circuit while it is connected to a source of voltage. Disconnect at least one end of the element being measured to avoid reading the resistance of parallel paths.
FIGURE 8-139. Shunt-type ohmmeter circuit.
movement and the limiting resistor. Therefore, when using an ohmmeter with a low-ohm scale, do not leave the switch in low-ohm position.
Use of the Ohmmeter The ohmmeter is not as accurate a measuring device as the ammeter or the voltmeter because of the associated circuitry. Thus, resistance values cannot be read with greater than 5 to 10 percent accuracy. While there are instruments which read the resistance of an element with very great accuracy, they usually are more complicated to use.
In addition to measuring the resistance, the ohmmeter is a very useful instrument for checking continuity in a circuit. Often, when troubleshooting electronic circuits or wiring a circuit, visual inspections of all parts of the current path cannot be readily accomplished. Therefore, it is not always apparent whether a circuit is complete or whether current might be flowing in the wrong part of the circuit because of contact with adjacent circuits.
The best method of checking a circuit under these conditions is to send a current through the circuit.
The ohmmeter is the ideal instrument for checking circuits in this manner. It provides the power and the meter to indicate whether the current is flowing.
Observe the following precautions when using an ohmmeter:
(1) Choose a scale which will contain the resistance of the element to be measured. In general, use a scale in which the reading will fall in the upper half of the scale (near full-scale deflection).
Megger (Megohmmeter) .
The megger, or megohmmeter, is a high-range ohmmeter containing a hand-operated generator.
It is used to measure insulation resistance and other high resistance values. It is also used for ground, continuity, and short-circuit testing of electrical power systems. The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential, or "breakdown" voltage. This type of testing ensures that insulation or a dielectric material will not short or leak under potential electrical stress.
The megger (figure 8-140) consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: (1) A hand-driven d.c. generator, G, which supplies the necessary current for making the measurement and (2) the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed-coil type. Coils A and B are mounted on the movable member with a fixed angular relationship to each other and are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The coils are mounted on a light, movable frame that is pivoted in jewel bearings and free to move about Coil A is connected in series with R3 and the R23 xis O I Hand generator Lero Ohmmeter FIGURE 8-140. Simplified megger circuit.
unknown resistance, Ra, to be measured. The series combination of coil A, R3, and Rz is connected between the + and - brushes of the d.c. generator. Coil B is connected in series with R2 and this combination is also connected across the generator.
There are no restraining springs on the movable member of the instrument portion of the megger.
When the generator is not in operation, the pointer floats freely and may come to rest at any position on the scale.
If the terminals are open-circuited, no current flows in coil A, and the current in coil B alone controls the movement of the moving element.
Coil B takes a position opposite the gap in the core (since the core cannot move and coil B can), and the pointer indicates infinity on the scale. When a resistance is connected between the terminals, current flows in coil A, tending to move the pointer clockwise. At the same time, coil B tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position at which the two forces are balanced. This position depends upon the value of the external resistance, which controls the relative magnitude of current of coil A.
Because changes in voltage affect both coil A and B in the same proportion, the position of the moving element is independent of the voltage. If the terminals are short-circuited, the pointer rests at zero because the current in A is relatively large.
The instrument is not damaged under these circumstances because the current is limited by R3.
There are two types of hand-driven meggers:
the variable type and the constant-pressure type.
The speed of the variable-pressure megger is dependent on how fast the hand crank is turned.
The constant-pressure megger utilizes a centrifugal governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant.
BASIC CIRCUIT ANALYSIS AND TROUBLE-
SHOOTING Troubleshooting is the process of locating causes for malfunctions or trouble in a circuit. The following definitions serve as a guide in the troubleshooting discussion:
(1) Short circuit-a low resistance path. It can be across the power source or between the sides of a circuit. It usually creates high current flow which will burn out or cause damage to the circuit conductor or components.
(2) Open circuit-a circuit that is not complete or continuous.
(3) Continuity-the state of being continuous, or connected together; said of a circuit that is not broken or does not have an open.
(4) Discontinuity-the opposite of continuity, indicating that a circuit is broken or not continuous.
Figure 8-141 includes some of the most common sources of open circuits (commonly called "opens" or "an open"). A loose connection or no connection is a frequent cause of an open circuit. In A of figure 8-141, the end of a conductor has separated from the battery terminal. This type of malfunction opens a circuit and stops the flow of current.
Another type of malfunction that will cause an open circuit is a burned-out resistor, shown in B of figure 8-141. When a resistor overheats, its resistance value changes; and, if the current flow through it is great enough, it can burn and open the circuit. In C, D, and E of figure 8-141, three more likely causes of open circuits are shown.
The opens shown can often be located by visual inspection; however, many circuit opens cannot be seen. In such cases, a meter must be used.
The circuit shown in figure 8-142 is designed to cause current to flow through a lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter can be used.
If a voltmeter is connected across the lamp, as shown in figure 8-143, the voltmeter will read zero.
Since no current can flow in the circuit because of the open resistor, there is no voltage drop across the lamp. This illustrates a troubleshooting rule that should be remembered: When a voltmeter is connected across a good (not defective) component in an open circuit, the voltmeter will read zero.
Next, the voltmeter is connected across the open resistor, as shown in figure 8-144. The voltmeter has closed the circuit by shunting (paralleling) the burned-out resistor, allowing current to flow.
Current will flow from the negative terminal of the battery, through the switch, through the voltmeter and the lamp, back to the positive terminal of the battery. However, the resistance of the voltmeter is so high that only a very small current flows in the circuit. The current is too small to light the lamp, but the voltmeter will read the battery voltage. Another troubleshooting point worth 332
Burned out Burned out Broken lamp bulb fuse wire D FIGURE 8-141. Common causes of open circuits.
remembering is: When a voltmeter is placed across an open component in a series circuit, it will read the battery, or applied voltage.
This type of open circuit malfunction can also be traced by using an ohmmeter. When an ohmmeter is used, the circuit component to be tested must be isolated and the power source removed from the circuit. In this case, as shown in figure 8-145, these requirements can be met by opening the circuit switch. The ohmmeter is zeroed and placed across (in parallel with) the lamp. In this circuit, some value of resistance is read. This illustrates another important troubleshooting point: when an ohmmeter is properly connected across a circuit component and a resistance reading is obtained, the component has continuity and is not open.
When the ohmmeter is connected across the open resistor, as shown in figure 8-146, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has been located with both a voltmeter and an ohmmeter.
An open in a series circuit will cause the current flow to stop. A short circuit, or "short," will cause the opposite effect. A short across a series circuit produces a greater than normal current flow. Some examples of shorts, as shown in figure 8-141, are two bare wires in a circuit that are touching each other, two terminals of a resistor connected together, etc. Thus, a short can be described as a Break Break FIGURE 8-143. Voltmeter across a lamp in an FIGURE 8-142. An open circuit. open circuit.
Break Break- FIGURE 8-144. Voltmeter across a resistor in an open circuit.
connection of two conductors of a circuit through a very low resistance.
In figure 8-148, a circuit is designed to light a lamp. A resistor is connected in the circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the current flow will increase and the lamp will become brighter. If the applied voltage were high enough, the lamp would burn out, but in this case the fuse would protect the lamp by opening first.
Usually a short circuit will produce an open circuit by either blowing (opening) the fuse or burning out a circuit component. But in some circuits, such as that illustrated in figure 8-149, there may be additional resistors which will not allow one shorted resistor to increase the current flow enough to blow the fuse or burn out a component. Thus, with one resistor shorted out, the circuit will still function since the power dissipated by the other resistors does not exceed the rating of the fuse.
To locate the shorted resistor while the circuit is functioning, a voltmeter could be used. When it is connected across any of the unshorted resistors, a portion of the applied voltage will be indicated on Break FIGURE 8-146. Using an ohmmeter to locate an open in a circuit component.
the voltmeter scale. When it is connected across the shorted resistor, the voltmeter will read zero.
The shorted resistor shown in figure 8-150 can be located with an ohmmeter. First the switch is opened to isolate the circuit components. In figure 8-150, this circuit is shown with an ohmmeter connected across each of the resistors. Only the ohmmeter connected across the shorted resistor shows a zero reading, indicating that this resistor is shorted.
The procedures used in troubleshooting a parallel circuit are sometimes different from those used in a series circuit. Unlike a series circuit, a parallel circuit has more than one path in which current flows. A voltmeter cannot be used, since, when it is placed across an open resistor, it will read the voltage drop in a parallel branch. But an ammeter or the modified use of an ohmmeter can be employed to detect an open branch in a parallel circuit.
If the open resistor shown in figure 8-151 was not visually apparent, the circuit would appear to be functioning properly, since current would continue to flow in the other two branches of the circuit. To determine that the circuit is not operating properly, the total resistance, total current, and the branch currents of the circuit should be calculated as if there were no open in the circuit:
RI = R N
- 3o = 10 Q total resistance.
FIGURE 8-145. Using an ohmmeter to check a Since the voltage applied to the branches is the circuit component. same and the value of each branch resistance is 334
Battery leads Resistor terminals Bare wires touching Improper wiring FIGURE 8-147. Common causes of short circuits.
known, An ammeter placed in the circuit to read total current would show 2 amperes instead of the calculated 3 amperes. Since 1 ampere of current should be flowing through each branch, it is obvious that one branch is open. If the ammeter is 30V 30V connected into the branches, one after another, = 30 8 30 g the open branch will be located by a zero ammeter reading.
= 1 ampere. = l ampere. A modified use of the ohmmeter can also locate this type of open. If the ohmmeter is connected Із - Е across the open resistor, as shown in figure 8-152, an erroneous reading of continuity would be obtained. Even though the circuit switch is open, 30V 30V the open resistor is still in parallel with R, and R2, = 30 ° = 10 S and the ohmmeter would indicate the open = 1 ampere. = 3 amperes (total current).
FIGURE 8-149. A short that does not open the FIGURE 8-148. A shorted resistor. circuit.
40 g 00 100 v 10 g 50g FIGURE 8-150. Using an ohmmeter to locate a shorted resistor.
resistor had a resistance of 15 ohms, the equivalent resistance of the parallel combination of Ri and R2.
Thus, it is necessary to open the circuit as shown in figure 8-153 in order to check the resistance of Rz- In this way the resistor is not shunted (paralleled) and the reading on the ohmmeter will indicate infinite resistance. On the other hand, if an open should occur in this circuit (figure 8-153) between the battery and point A, or between the battery and point B, current would not flow in the circuit.
As in a series circuit, a short in a parallel circuit will usually cause an open circuit by blowing the fuse. But, unlike a series circuit, one shorted component in a parallel circuit will stop current flow by causing the fuse to open. This can be seen by. referring to the circuit in figure 8-154.
If resistor R, is shorted, a path of almost zero resistance will be offered the current, and all the circuit current will flow through the branch containing the shorted resistor. Since this is practically the same as connecting a wire between the terminals of the battery, the current will rise Rz 302 300 308 3 Break FIGURE 8-152. A misleading ohmmeter indication.
to an excessive value, and the fuse will open. Since the fuse opens almost as soon as a resistor shorts out, there is no time to perform a current or voltage check. Thus, troubleshooting a parallel d.c.
circuit for a shorted component should be accomplished with an ohmmeter. But, as in the case of checking for an open resistor in a parallel circuit, a shorted resistor can be detected with an ohmmeter only if one end of the shorted resistor is disconnected.
Troubleshooting a series-parallel resistive circuit involves locating malfunctions similar to those found in a series or a parallel circuit.
In the circuit shown in figure 8-155, an open has occurred in the series portion of the circuit. When an open occurs anywhere in the series portion of a series-parallel circuit, current flow in the entire circuit will stop. In this case, the circuit will not function, and the lamp, Li, will not be lit.
If an open occurs in the parallel portion of a series-parallel circuit, as shown in figure 8-156, part of the circuit will continue to function. In this case, the lamp will continue to burn, but its brightness will diminish, since the total resistance Open 30 v Rz 300 Break 3025 300 3 2 a Break FIGuRE 8-153. Opening a branch circuit to FIGURE 8-151. Finding an open branch in a obtain an accurate ohmmeter parallel circuit. reading.
30 v 300 300 300 FIGURE 8-154. A shorted component causes the fuse to open.
of the circuit has increased and the total current has decreased.
If a break occurs in the branch containing the lamp, as shown in figure 8-157, the circuit will continue to function with increased resistance and decreased current, but the lamp will not burn.
To explain how the voltmeter and ohmmeter can be used to troubleshoot series-parallel circuits, the circuit shown in figure 8-158 has been labeled at various points. By connecting a voltmeter between points A and D, the battery and switch can be checked for opens. By connecting the voltmeter between points A and B, the voltage drop across Ri can be checked. This voltage drop is a portion of the applied voltage. Also, if R, is open, the reading between B and D will be zero. The conductor between the positive terminal of the battery and point E, as well as the fuse, can be checked for continuity by connecting the voltmeter between points A and E. If the conductor or fuse is open, the voltmeter will read zero.
If the lamp is burning, it is obvious that no open exists in the branch containing the lamp, and the voltmeter could be used to detect an open in the branch containing R2 by removing lamp, L1, from the circuit.
Troubleshooting the series portion of a seriesparallel circuit presents no difficulties, but in the Break Break FIGURE 8-156. An open in the parallel portion of a series-parallel circuit.
parallel portion of the circuit misleading readings can be obtained.
An ohmmeter can be used to troubleshoot this same circuit. With the switch open, the series portion of the circuit can be checked by placing the ohmmeter leads between points A and B. If R1 or the conductor is open, the ohmmeter will read infinity; if not, the value of the resistor will be indicated on the ohmmeter. Between points D and E the fuse and conductor can be checked for continuity, but in the parallel portion of the circuit, care must be exercised, since misleading ohmmeter indications can be obtained. To check between points B and E, the branch must be disconnected at one of these points, and while one of these points and the switch are open, the branch containing the lamp can be checked with the ohmmeter.
A short in the series part of a series-parallel circuit will cause a decrease in total resistance, which will cause total current to increase. In the circuit shown in figure 8-159, the total resistance is 100 ohms and the total current is 2 amperes. If RI became shorted, total resistance would hecome 50 ohms, and the total current would double to 4 amperes. In the circuit shown, this would cause the 3-amp fuse to blow, but with a 5-amp fuse the circuit would continue to function. The result would be the same if Ra or R3 were to become shorted. The total resistance in either case would m Break FIGURE 8-155. An open in the series portion of a FIcURE 8-157. An open lamp in a series-parallel series-parallel circuit. circuit.
E F FIGURE 8-158. Using the voltmeter to troubleshoot a series-parallel circuit.
drop to 50 ohms. From this, it can be stated that when a short occurs in a series-parallel circuit, the total resistance will decrease and the total current will increase. A short will normally cause an open circuit by either blowing the fuse or burning out a circuit component. And, as in the case of an open, a short in a series-parallel circuit can be detected with either an ohmmeter or a voltmeter.
ALTERNATING CURRENT AND VOLTAGE Alternating current has largely replaced direct current in commercial power systems for a number of reasons. It can be transmitted over long distances more readily and more economically than direct current, since a.c. voltages can be increased or decreased by means of transformers.
Because more and more units are being operated electrically in airplanes, the power requirements are such that a number of advantages can be realized by using a.c. Space and weight can be saved, since a.c. devices, especially motors, are smaller and simpler than d.c. devices. In most a.c.
motors no brushes are required, and commutation trouble at high altitude is eliminated. Circuit breakers will operate satisfactorily under load at high altitudes in an a.c. system, whereas arcing is so excessive on d.c. systems that circuit breakers must be replaced frequently. Finally, most airplanes using a 24-volt d.c. system have special equipment which requires a certain amount of 400-cycle a.c. current.
A. C. and D. C. Compared Many of the principles, characteristics, and effects of alternating current are similar to those of direct current. Similarly, there are a number of differences which are explained later. Direct current flows constantly in only one direction with a constant polarity. It changes magnitude only when the circuit is opened or closed, as shown in the d.c. wave form in figure 8-160. Alternating current changes direction at regular intervals, increases in value at a definite rate from zero to a maximum positive strength, and decreases back to zero; then it flows in the opposite direction, similarly increasing to a maximum negative value, and again decreasing to zero. D.C. and a.c. wave forms are compared in figure 8-160.
Since alternating current constantly changes direction and intensity, two effects take place in
a.c. circuits that do not occur in d.c. circuits. They are inductive reactance and capacitive reactance.
Both are discussed later in this chapter.
Generator Principles After the discovery that an electric current flowing through a conductor creates a magnetic field around the conductor, there was considerable scientific speculation about whether a magnetic field could create a current flow in a conductor. In 1831, the English scientist, Michael Faraday, demonstrated this could be accomplished. This discovery is the basis for the operation of the generator, which signalled the beginning of the electrical age.
To show how an electric current can be created by a magnetic field, a demonstration similar to that illustrated in figure 8-161 can be used.
Several turns of a conductor are wrapped around a cylindrical form, and the ends of the conductor are connected together to form a complete circuit which includes a galvanometer. If a simple bar magnet is plunged into the cylinder, the galvanometer can be observed to deflect in one direction from its zero (center) position (4 of figure 8-161).
When the magnet is at rest inside the cylinder, the 500 Rz 200 v 10050 1, = 2 Amps FIGURE 8-159. Finding a short in a series-parallel circuit.
Wave form for DC Wave form for AC VOLTS VOLTS 180° 270° 360° 0° 90° Operation of circuit Closed ' Open Switch Switchi Time - Time FIGURE 8-160. D.C. and a.c. voltage curves.
galvanometer shows a reading of zero, indicating that no current is flowing (B of figure 8-161).
In C of figure 8-161, the galvanometer indicates a current flow in the opposite direction when the magnet is pulled from the cylinder.
The same results may be obtained by holding the magnet stationary and moving the cylinder over the magnet, indicating that a current flows when there is relative motion between the wire coil and the magnetic field. These results obey a law first stated by the German scientist, Heinrich Lenz. Lenz's law states that the induced current caused by the relative motion of a conductor and a magnetic field always flows in such a direction that its magnetic field opposes the motion.
When a conductor is moved through a magnetic field, as shown in figure 8-162, an electromotive force (e.m.f.) is induced in the conductor. The direction (polarity) of the induced e.m.f. is determined by the magnetic lines of force and the direction the conductor is moved through the magnetic field. The generator left-hand rule (not to be confused with the left-hand rules used with a coil) can be used to determine the direction of the induced e.m.f., as shown in figure 8-163. The first finger of the left hand is pointed in the direction of the magnetic lines of force (north to south), the thumb is pointed in the direction of movement of the conductor through the magnetic field, and the second finger points in the direction of the induced Motion of magnet Galvanometer
- Coil Motion of magnet Magnet at Rest B FIGURE 8-161. Inducing a current flow.