VI Applications of the Electro-Magnet
It became quite the rage now among the boys to find as many uses of electro-magnets as possible. These were reported and explained to the club and a list kept. This list included:
1. Dynamo.
2. Magneto.
3. Ammeter.
4. Wattmeter.
5. Motor.
6. Electric piano and organ players.
7. Electric door openers.
Already noticed in the preceding pages, and the following:
8. The Electric Spinner (Fig. 26).—A toy full of instruction. The standard is a steel magnet which produces a magnetic field. Inside of this is an electro-magnet which serves as an armature. Plainly visible on its shaft is a commutator to which the electric current from a dry cell is sent. This causes the armature to revolve and carry with it a series of colour disks which may be adjusted so as to show what tint or shade results from mixing colours in various proportions.
9. The Electric Engine (Fig. 27).—This toy, with one dry battery cell, develops power enough to run several other toy machines. The diagram in Fig. 28 will make its plan of operation plain. B is the battery cell, c the electro-magnets, a an armature of iron. By a rod this armature is connected with a crank on the axle which carries the fly wheel f. Another crank, d, upon the same axle serves like a push button to close the electric circuit at the right instant. The wire g from the battery cell encircles the electro-magnet c and then is connected to the iron base of the toy. When the crank d touches the conductor e, which is a spring, the electric current passes around the magnet, the magnet pulls the iron armature a, and this gives an impulse to the wheel f whose momentum carries it around during that portion of the revolution when d is separated from e and a is receding from the magnet.
It is customary to say that the circuit is closed through the base of the machine, but this language requires interpretation. It means that a way is provided for the electric current to pass through the base. A person who is expert in language but not in electricity might expect us to say “the circuit is open through the base.”
10. The Telegraph Sounder (Fig. 29).—This was a toy half a century ago, but since the days of Samuel Finley Breese Morse it has become of vast commercial importance. The Western Union Telegraph Company in 1909 had 211,513 miles of poles and cables, 1,382,500 miles of wire, 24,321 offices, sent 68,053,439 messages, received $30,541,072.55, expended $23,193,965.66, and had $7,347,106.89 in profits. In the United States more than 93,000,000 and in the world at large more than 600,000,000 messages are sent annually, and there are men still living who scoffed at Morse’s ideas as impracticable.
It is interesting to contemplate what would happen to the Stock Exchange, to the newspapers, to the railroads, to the congressman addressing his constituents from the floor of a legislative chamber, to business in general, if the world were deprived of the telegraph.
A few years ago a telegraph despatch was sent from New York to San Francisco, Tokio, London, and back to New York, 42,872 miles, in three minutes less than an hour. Electricity can travel around the world in a fraction of a second, the time was consumed in repeating the message. I once sent a message from New York to New Haven to announce that I was coming, and afterward took my train and reached New Haven in time to receive my own message and pay the messenger boy. But I have never lost faith in the beneficent results of Morse’s labours.
Morse (1791–1872) was an artist and the first President of the National Academy of Design. He was likewise a professor in New York University and constructed his first experimental telegraph line upon the University campus in 1835. His first public line was built from Washington to Baltimore in 1844. The Western Union Telegraph Company was incorporated in 1856. Of course the work of Morse rested upon that of Oersted, in Copenhagen, who, in 1819, discovered electro-magnetism, and upon that of Joseph Henry of Albany, who in 1827 first insulated the wires.
The application of the electro-magnet to producing telegraphic signals will be understood by referring to Fig. 30. B is the generator of an electric current—sometimes a battery and sometimes a dynamo. One wire from this goes to the earth, E. The other wire goes through a key, which, like a push button or a switch, serves to open or close the circuit. This is normally closed when not in use. Through this the current passes around the electro-magnet S, which attracts the armature a, causing it to click against a metal stop, hence it is called the sounder. From this the current passes along the line wire to a distant station and there through the sounder and closed key to the earth. There is likely to be a generator at each station. The current must run continually through the system. If a battery is employed, the copper sulphate, or gravity cell, to be described later, is chosen, because it will endure continued usage better than any other.
The operator, in sending signals, opens the circuit, the magnets cease to hold down the armatures, and they are raised by springs and strike against metallic stops above. It is customary to say that the circuit is completed through the earth. This statement misleads some persons into imagining an electric current capable of corroding water pipes and decomposing chemical compounds, passing through the earth between stations.
Perhaps it will help to a better understanding of the truth if we think of a city pumping water out of the ocean, say to fight fire, and disposing of it again into the ocean. The ocean currents thus produced are not likely to be destructive. Indeed, just as we measure height from the ocean level as zero, so we measure electric pressures as from the zero level of the earth’s electrical state.
The key used by telegraphers is represented in Fig. 31. It has connected with it a switch to keep the circuit closed when the key is not in operation. The Morse code of signals consists of dots and dashes, when printed, as follows:
| a | . - |
| b | -... |
| c | ... |
| etc. |
Operators learn to read the message by the intervals between sounds. A dot consists of two taps of the sounder with a short interval between, and a dash consists of two taps with a longer interval between. One tap of the sounder is caused by its descending upon the metal stop below and another by its rising against the upper stop.
Telegraph sounders are operated on about a quarter of an ampere of current if from a battery circuit, or on about one tenth of an ampere from a dynamo circuit. The dynamo circuit is supplied with more volts of electric pressure, and hence its power is ample to cause the armature to strike the metal stops hard enough to be heard by the operator.
For example a battery circuit may supply to the sounder a current with these characteristics:
2 volts ×.25 amperes =.5 watts, while a dynamo circuit may give:
6 volts ×.1 ampere =.6 watts.
Telegraph line wires are usually bare, the insulation being merely the glass knobs at the poles. Clean water is a very good insulator but dirty water is a fairly good conductor. A wet telegraph pole may bring so much current to earth as to prevent all sounders on the line from operating. Hence the line is separated from the poles by glass. The poles are about one hundred and thirty-two feet apart, making forty to the mile. The wires are usually galvanized iron one sixth of an inch in diameter. Copper conducts six times as well as iron, and is now replacing iron in the lines.
Morse laid a submarine telegraph line in New York Harbour and suggested a cable across the ocean. But that gigantic undertaking had to await the masterful intelligence of Lord Kelvin and the indomitable will of Cyrus W. Field. A submarine cable was laid across the Strait of Dover in 1850. It was cut by the anchor of a fisherman a few hours after it was laid. The first attempt to lay a submarine cable across the Atlantic Ocean was made in 1857. Two ships of war, the Agamemnon of Great Britain and the Niagara of the United States, engaged in this undertaking. Three hundred miles had been laid when the cable parted where the ocean was more than two miles deep. William Thomson was on board the Agamemnon as electrical expert. He went home to study and improve the methods. The next year, 1858, the Agamemnon and the Niagara met in midocean each with a portion of the cable on board. The splice was made, and the Agamemnon started toward Ireland and the Niagara toward Newfoundland. When six miles apart the cable broke. The ships met again, made a new splice and again started in opposite directions. They laid eighty miles and the cable parted a second time. They met again, spliced and laid two hundred miles when it parted for the third time. They met a fourth time, made the splice and succeeded in laying the first cable from Ireland to Newfoundland on August 5, 1858.
In a few weeks the insulation failed and no more messages could be sent. Seven years were spent in studying the problem, and again in 1865 the Great Eastern, a mammoth ship, started to lay the cable. William Thomson was again on board as the expert. When twelve hundred miles had been laid the cable parted in deep water. Three times the cable was grappled and brought part way to the surface and lost again. The Great Eastern returned to land. The next year, 1866, the Great Eastern, having on board William Thomson (Lord Kelvin), Mr. Canning, the engineer of the expedition, and Captain Anderson, in command, laid the cable which has worked successfully ever since. Thomson, Canning, and Anderson were knighted as a result of their labours. Sir William Thomson (1824–1907), afterward Lord Kelvin, is credited with having solved the difficult electrical problems connected with this enterprise. Cyrus W. Field (1819–1892), born in Stockbridge, Mass., helped to secure the many millions of dollars necessary to carry the work to completion.
There are now seventy-three cables connecting Europe and America, and two across the Pacific Ocean. Cable rates are: New York to England, France, Germany, or Holland twenty-five cents a word, to Switzerland thirty cents a word, and to Japan one dollar and thirty-three cents a word.
The boys were kept very busy now looking up historical and biographical sketches, as well as working up the many applications of the electro-magnet. The next to be reported was:
11. The Relay (Fig. 32).—Telegraphing from 3,000 to 10,000 miles under the ocean is full of difficulties not now to be explained.
Of course when we attempt to telegraph many miles upon land we find that the resistance of the wire cuts down the strength of the current so that it will not move the sounder. This, however, is readily obviated by the relay devised by Morse. It simply serves as an automatic key to close a circuit. A diagram will make this clear (Fig. 33). Suppose the line wire to be very long and on account of its resistance the current is too feeble to operate a sounder. It is likely to be about.025 ampere where the local sounder may require.25 ampere or ten times as much. It is easily possible to wind a magnet (Fig. 33), R, such that.025 ampere will close the armature a, so that it may complete a local circuit when it would not make noise enough for a sounder. B may represent a local battery of any desired strength which may operate the sounder S of that station as loudly as may be desired.
12. Annunciator (Fig. 34).—We live in a fifth-floor apartment. When we push the button to call the elevator a No. 5 appears in the annunciator in the elevator car. This tells the elevator boy where the call comes from. Take out two or three screws and the annunciator opens, revealing a series of electro-magnets like the one shown in Fig. 35. When an electric current passes around the coil it pulls back an iron catch and allows a number to drop so as to show through a small window. The elevator boy, having noted that the call is from the fifth floor, pushes up the number and the iron catch holds it until the coil is magnetized again by an electric current.
The annunciator has a bell to call attention. A cable of six wires enters this annunciator (Fig. 36). One wire goes direct to the bell and the other five reach the bell through the separate coils of the electro-magnets which control the drops. But how are electrical connections made between a moving elevator car and the push buttons on various floors? The diagram in Fig. 37 shows this in elevation. B represents a battery of several dry cells located in the basement. One wire from it runs direct to the push buttons 1, 2, 3, 4, 5, located upon the five floors of the house. The other wire from the battery, together with wires from each of the five push buttons, all run to a point, A, half-way up the elevator shaft. Here the six wires are gathered into a cable long enough to reach either to the top or the bottom of the elevator shaft. The other end of this cable enters the elevator car and runs to the annunciator. The wire from the battery goes direct to the bell. The wires from the various push buttons go through correspondingly numbered electro-magnets to the bell. When, therefore, we pushed the button on the fifth floor, we closed the gap in the electric circuit at that point. The current came up from the battery, passed through the button, went down the cable to the car, went through electro-magnet No. 5, went through the bell, and returned direct to the battery, thus completing the circuit. Annunciators are used about buildings to call other attendants, besides the elevator boy. They are likewise used in burglar alarms to inform the householder which door or window is being forced. They are used in the fire department to tell what part of the city the call came from.
13. The Electric Bell and Buzzer (Fig. 38).—So common a thing as an electric bell really belongs to the present generation. Bells were either novelties or toys when I was your age. They cost then many times what they do now and then were poorly made. Nobody dared to trust them for front-door bells. It was necessary to have a card permanently posted over the push button saying, “If the bell does not ring, knock.” In those days batteries were troublesome to care for, houses were not wired when built, and no one had learned the art of concealing the wires neatly.
The buzzer is simply a bell minus gong and hammer. Those shown in Fig. 38 ring well on a single dry cell. A cell costing twelve cents operated one for two years while it was used as a call bell from dining room to kitchen, the current required being.15 ampere.
The connections are shown in the diagram (Fig. 39). Suppose the current to enter at the binding post a, pass around the magnets b and then to the post c. The armature d normally rests against the post c and the current finds its way along this to the post e and thence back to the battery. But as soon as the current passes, b becomes a magnet and pulls the armature d away from the post c, thus breaking the circuit, when b ceases to be a magnet and a spring pushes the armature d back against the post c to repeat the operation. The armature d carries a hammer which strikes the gong f. If the wire, which is usually connected with the binding post e, is connected with the post c, the “clatter” bell is changed to a “single-stroke” bell, and if the gong and hammer are removed the “bell” is changed to a “buzzer.”
In the case of the buzzer, by changing the length of the armature or by weighting it, we may change the time of its vibrations and its tone. The connections between battery push button and bell form a complete circuit. In Fig. 40 B represents a battery, usually of dry cells, B’ represents the bell, and P represents the push button. The electric circuit is “open,” (that is, there is a break in the conductor) at P until some one “pushes the button,” that is, simply pushes against a spring so as to cause a piece of metal to bridge the gap in the conductor. Then we say the circuit is “closed.”
Push button devices and switches are innumerable. In every case they are simply devices for pushing one piece of metal against another and completing the circuit for an electric current. Every one should unscrew and examine a few of them, both for the pleasure of seeing how they work and to learn how to make them work when they sometimes fail. Not only in bells but in all other instruments where electro-magnets are used, the magnets are placed in pairs, fastened together upon an iron base. They are wound so that the free ends are made opposite poles by the electric current. Like a horseshoe magnet, they form one magnet. The two poles thus placed are mutually helpful and each is stronger than it would be if separated from the other.
14. Electric Clocks, Self-winding Clocks, Programme Clocks.—A pretentious-looking thing which appeared like a dish pan with a glass bottom was opened by the boys and found to be the simplest of all clocks. It had an electro-magnet like that in Fig. 44. A strip of iron acting as an armature across the free ends of this magnet, pushed like a finger against the cogs of a wheel. This wheel was on the axle of the minute hand and it had sixty cogs. The electric circuit was closed through the magnet for an instant each minute and the armature pushed the wheel ahead one cog. Thus it made one complete revolution in an hour. A train of four other cog-wheels caused the hour hand to trail after at one twelfth the speed of the minute hand. This machinery made simply a small handful in an eighteen-inch stamped-metal “dish-pan” costing fifteen dollars.
A self-winding clock was opened and found to contain two dry battery cells, an electro-magnet which operated very much like that of a “clatter” bell, the hammer like a finger poking against the cogs of a wheel. Once an hour the long hand closed the circuit through the battery and the magnet and its armature swung back and forth long enough to give the cog wheel one complete revolution and wind a spring, which it carried upon its axle. This spring kept the clock running one hour, until the next winding.
The programme clocks which were examined were self-winding clocks, but were connected by wires to the master clock which corrected them each hour. Each time the long hand of the master clock came to twelve it closed an electric circuit through all the clocks in the system. In each clock the current passed around an electro-magnet and caused it to pull an armature against a metal stop and set each long hand exactly at twelve. This master clock is sometimes situated many miles away and may correct the time for a whole city. Thus a master clock at Washington, D. C., furnishes standard time to all parts of the United States. The master clock which we examined also closed the circuit at proper intervals through a series of programme bells placed in the various class rooms, and these called and dismissed classes automatically.
15. Watchman’s Time Detector (Fig. 45).—This is a device to compel a watchman to make his appointed trips. Push buttons or switches are distributed about the building at various points, and it is made his duty to close the circuits at these points at stated times. When he does so, the fact is recorded by electro-magnets puncturing, or, in some way, marking a revolving time card in the clock.
16. Circuit Breakers (Fig. 46).—Electro-magnets are used to open switches and thus protect dynamos and other machines against a larger electric current than they are able to carry. The switch is held closed by a spring which, by an adjusting device, may be tightened or loosened. A dynamo which we examined had its circuit breaker adjusted so that it would remain closed if any current under 1500 amperes passed, but if a greater current than that passed it would strengthen the magnet sufficiently to open the switch and thus break the circuit.
17. Separating Iron from Ore.—In 1897 Edison first proposed to use an electro-magnet to separate iron from crushed earth. Fig. 47 represents the process. E is an electro-magnet. S is the stream of crushed ore containing iron. Gravity would cause all the material to fall into bin A, but the electro-magnet E pulls that portion of the material which is magnetic to one side so that it falls into the bin B.
18. Lifting Magnets.—Electro-magnets are made for use with hoisting apparatus to save the trouble of manipulating grappling hooks, etc. They may lift barrels and boxes of iron, the wood of the barrel or box being transparent, we say, to the magnetic influence. That is, the magnet will attract iron through the wood just as light will shine through glass. Such magnets are used to pick up from the bottom of the sea cases of hardware from wrecked ships. (See the accompanying illustration, Fig. 48.) In such cases the electric conductors which lead to and encircle the magnets must be well insulated from the water of the sea, otherwise the electric current would take the shorter path from one line wire through the sea water, which is a fairly good conductor, and back by the other line wire, rather than go the path of greater resistance around the magnet. Electro-magnets are coming into use in foundries, etc., for lifting heavy iron castings.
19. Electro-Magnet on Starting Box.—As was explained under electric motors, a starting box is simply a series of resistance coils r, r, r, r, r, in Fig. 49. When the motor is not in use the switch l rests upon the point 1 and no electric current passes. When the switch is moved to point 2, the current entering at a passes to the pivot of the switch and up the metal strip l to the point 2, then around the series of coils, r, r, r, r, r, to the post b and thence back to the generator. As the switch is moved to the right, the current passes through less and less of this resistance until, when it reaches point 7, all the coils of resistance are “cut out,” that is, they are not in the path of the current. Now the motor has reached its full speed and is developing enough counter-electro-motive force to protect itself against too much current. Through a shunt, however, a portion of the current passes from a to b around the electro-magnet e, the two poles of which are presented to the metal strip l, which must be of iron. This magnet holds the switch over so long as the current is on, but when the current is cut off, by opening a switch in the line wire, e ceases to be a magnet and l is carried back to point 1 by a spring. Thus an extra resistance must always be in circuit when the motor is first started. Those who start motors are expected to move the lever l of the starting box slowly from point to point, pausing a second or two on each to give the motor time to acquire proper speed for its protection. How too great a current would “burn out” a motor will be explained later.
The motor man handles a lever for starting his car, which works like that of the “starting box.” His “starting box,” however, is called a “controller.” Although it accomplishes the same result as the starting box it has a wholly different and vastly more complex mechanism than that already described.
The elevator boy, who runs our electric elevator, handles a lever which also does the same thing through far different mechanism. Indeed, in his case electro-magnets are used to prevent him from cutting out resistance too fast if he should move his lever too quickly.
20. Starting Switches for Electric Elevators.—The motor man has to be instructed particularly how he should handle the lever of his controller, and he is trusted to follow his directions to some extent, however lacking in intelligence and integrity he may be. But the elevator boy receives scarcely any instructions about his machine, and, indeed, his machine has been constructed pretty nearly “foolproof.” It will automatically correct his errors of management. If he throws the handle from one extreme to the other, all resistance cannot be thrown out instantly, but this is accomplished by a series of electro-magnets closing one switch after another and thus cutting out resistance gradually.
21. Arc Lamp Feed.—As will be explained later, an arc lamp must have its carbons touching one another when the current is first thrown on, and then the carbons must be drawn apart from a quarter to half an inch. The upper carbon is lifted away from the lower one by a portion of the current passing by means of a shunt around an electro-magnet.
22. Volt meter.—The volt meter measures the pressure of an electric current. The volt meter which we examined looked outside like our ammeter, and when we removed the face it appeared inside like an ammeter. There was the steel magnet of horseshoe shape to furnish a field (Fig. 51), and there was an electro-magnet poised between its poles for an armature. The armature in the volt meter, however, had wound upon it finer wire and more of it than was the case in the ammeter. There was no shunt wire in the volt meter as there was in the ammeter. We connected in series a fluid cell (to be described later), the ammeter, and the volt meter (Fig. 52). The ammeter shunt was removed so that all the current went through its armature. The volt meter needle went to one which was two thirds of the scale (Fig. 53), and the ammeter needle indicated.016. That is, this particular cell can push sixteen thousandths of an ampere through the resistance of this volt meter, and.016 ampere passing through the armature of this volt meter will magnetize it sufficiently to move it against its spring, say sixty degrees.
We put into the circuit a lot more fine wire for resistance, R (Fig. 54), so that the volt meter needle went only half as far as before, that is to.5. The ammeter indicated only half as much as before, that is.008 ampere. We put in resistance enough to bring the volt meter needle down to.25 and the ammeter indicated one quarter of the original current. We put in less resistance, bringing the volt meter needle to.75, and the ammeter indicated three fourths of the original current. Evidently the volt meter is merely an ammeter with a different scale marked upon its card. With a pen we marked upon the card of the volt meter a true ammeter scale (Fig. 55).
In order to understand the volt meter, let us turn our attention for a moment to Fig. 56. I have arranged the water tank T at such a height above the faucet F that when the faucet is opened one quart of water will flow in a minute. If I partially close the faucet, making the opening one half as large (that is, offering twice the resistance to the flow), half a quart will flow in a minute. If I make the resistance four times as great only one quarter of a quart will flow in a minute. It is evident that I could arrange a scale underneath the handle of the faucet to indicate the quantity of water flowing, just as the ammeter and volt meter indicate the quantity of electricity which flows. If now that much is understood, it will be easy to learn how the water faucet may be used to measure water pressure and the volt meter in like manner used to measure electric pressure.
Having set the faucet so that a quart will flow per minute, let us put on a longer tube p, and move the tank up to another shelf so that the distance from the water level in the tank to the faucet is twice as great as before. Under the increased pressure water runs through the faucet twice as fast and we now get two quarts per minute.
I purposely placed the tank out of sight behind a partition so that you might practise judging the water pressure by the flow at the faucet. We cannot very well talk about pressure in quarts. We might talk about it in pounds, but if we used this apparatus much we should probably get into the habit of talking about the pressure from one shelf, two shelves, three shelves, etc.
In order that the pressure might remain nearly constant during the experiment we would probably introduce resistance (that is, partially close the faucet) so that the water level should not fall much. We might, for example, set the faucet so that half a pint would flow in a minute when the tank was on the first shelf. Then a pint per minute would flow when the tank was on the second shelf and one and a half pints per minute when the tank was on the third shelf, etc. Thus we should infer the pressure by measuring the quantity.
One more illustration and the case will be clear. To save the trouble of measuring the quantity of water which flows through the faucet, suppose I introduce the device represented in Fig. 57. W is a small water wheel comparable to the armature of the volt meter. It carries a pointer which moves over a scale just as in the case of the volt meter.
It has a spring coiled around its axle which tends to keep the pointer at 0, as in the case of the volt meter. The tank is placed upon the first shelf, the faucet is fixed so that a small amount of water flows and the needle moves to a certain figure upon the scale. We will mark this point one and call it “first-shelf pressure.” The tank is lifted to the second shelf and the index moves to another point, which we will mark two and call it “second-shelf pressure.” The tank is lifted to the third shelf and the index moves to a third point, which we will mark three and call it “third-shelf pressure,” etc.
Ordinarily we measure water pressure with an instrument which allows no water to run to waste, but in measuring electric pressure by the volt meter some current must pass through the instrument, just as in the case of our water-wheel illustration in Fig. 57. We put in large resistance so as to make this current as small as possible, while we let enough pass to move the armature.
Now let us return to the volt meter itself. By referring to Fig. 55, we see that it requires.024 ampere to move the needle of the volt meter clear across the scale, and we have found that one fluid cell was able to send enough current through the resistance of the armature to move the needle two thirds of the way across the scale. At this point we find Fig. 1, which might be read “one-cell pressure.” We prefer to commemorate the name of one of the workers in the field of electricity and call this pressure a “volt” after Alessandro Volta (1745–1827), born at Como, Italy. It is the electric pressure which is produced by one fluid cell of a certain kind. We say, then, that one volt pushes through the resistance of this armature.016 ampere. Half a volt would push through the resistance of the armature half as much current or.008 ampere. At this point we put.5. Thus each of the figures in the lower row (Fig. 55) shows what part of a volt is required to send enough current through this particular armature to move the needle to that point.
We found out how much wire was wound upon the armature and put exactly the same amount in the outside resistance, R (Fig. 59). The needle now showed that one volt is able to push through twice the resistance of the armature only half as much current, and the needle stopped at.008 ampere. If this were to be the resistance in the volt meter circuit one volt should stand under.008 ampere and two under.016 and three under.024. It is evident then, that, if we know the internal resistance of a volt meter, we may make it capable of measuring greater electrical pressures by adding the proper amount of resistance. By putting at R, (Fig. 59) nine times the internal resistance of the instrument, thus multiplying the total resistance tenfold, the figures upon the scale of volts may be read as whole numbers from one to fifteen. In this case it will require fifteen cells to push the needle clear across the scale and ten cells to push it two thirds of the way across. If now we add enough external resistance to multiply the resistance of the armature a hundred fold it will require 150 volts to push.024 of an ampere through the armature and pull its needle clear across the scale. In this case the figures upon the scale of volts are multiplied by one hundred and read from ten to one hundred and fifty. Such a scale would adapt this volt meter for use with our 110-volt lighting circuit. Volt meters are made with a series of such external resistances, called “multipliers,” attached so that they may be easily thrown into the circuit.
It is evident that we need some term so that we may speak of quantities of resistance. This need has given rise to a unit of resistance called an ohm, after George Simon Ohm (1789–1854) born at Erlanger in Bavaria. Two inches of No. 36 German silver wire, such as is wound upon the armature of this volt meter, gives one ohm of resistance. There are 125 inches of this wire upon the armature. Its resistance is, therefore, 62.5 ohms, and we may, therefore, say that one volt of electric pressure can push through 62.5 ohms of resistance.016 of an ampere of current. Ohm discovered this relationship in 1827, and formulated it as follows:
volts/ohms = amperes (not, however, using these words).
(1 volt)/(62.5 ohms) =.016 ampere.
| 62.5) | 1.0000 | (.016 |
| 625 | ||
| —— | ||
| 3750 | ||
| 3750 | ||
| —— |
This is called Ohm’s law, as every candidate for college admission will hear and hear again.
Volt meters and armatures for the alternating current have electro-magnets for their fields as well as for their armatures. Such instruments are equally well adapted for either direct or alternating currents. For when the current reverses its direction it reverses in field and armature alike, and thus a repulsion between like poles is maintained. Such an instrument, however, cannot respond to as slight a current as those previously described, since they must consume some energy in both field and armature.
23. Telephone Receiver (Fig. 61).—It requires a stretch neither of the imagination nor of the truth to call a telephone receiver an electro-magnet, although perhaps it has never been called that before. We took it apart and found that it consisted of a steel-bar magnet m (Fig. 62), with a small spool of wire w around one end of it. The ends of the wire on the spool run along inside the hard rubber shell to the two binding posts a and b at the other end. A disk of sheet iron S is held in the large end of the case very near to, but not quite touching, the end of the magnet. When an alternating current is sent through the wire upon the spool it causes rapid changes in the strength of the magnetic field, if not reversals of the poles of the field, and the iron disk is made to vibrate, keeping time with the alternations of the current.
In this laboratory we have seen that our current has sixty alternations per second. When it is connected with the receiver the disk, therefore, makes sixty vibrations per second, and produces a tone which has very nearly the pitch of C two octaves below the middle C upon the piano.
24. Spark Coil (Fig. 63).—The automobile spark coil which we have already used is an electro-magnet. The battery sends a current through wire coiled around an iron core. At one end of this iron core is an iron armature which is made to vibrate in precisely the same manner as the armature of an electric bell. This makes and breaks the current and causes rapid changes in the strength of the field. A rapidly changing magnetic field may be used to develop electricity in a conductor, as we have already seen in the case of the dynamo.
How it is used in the automobile spark coil will be shown later. It is sufficient now to mention it as a case of a magnetic field produced by an electric current passing through a wire coiled around an iron core, or, in short, an electro-magnet.
Induction coils, Ruhmkorff coils, and transformers, to be described later, are closely related to this. They all create magnetic fields in the same way and are all electro-magnets.