Electro-Magnetism
What is electro-magnetism?
It is the magnetism developed through the agency of electrical or galvanic action.
What were the earliest phenomena observed which indicated a relation between magnetism and electricity?
It was noticed that ships’ compasses have their directive power impaired by lightning, and that sewing needles could be rendered magnetic by electric discharges passed through them.
What discovery, made by Prof. Oersted of Copenhagen, established beyond a doubt the connection of electricity and magnetism?
He ascertained that a magnetic needle placed near a metallic wire connecting the poles of a galvanic battery was compelled to change its direction, and that the new direction it assumed was determined by its position in relation to the wire and to the direction of the current transmitted along the wire.
Thus, if a needle be inclosed in a wire not touching it at any point, and a current of electricity pass through the wire, the needle will be made to move in accordance with the direction of the current.
What other important discovery was made about the same time?
It was found that if a piece of soft iron, not possessing magnetic power sufficient to elevate a grain weight, be placed within a coil of copper wire through which a galvanic current is passing, it will become, through the influence of the current, a powerful magnet; and will, so long as the current flows, sustain weights amounting to many hundreds of pounds.
Is the magnetic power of the bar found to be wholly dependent on the existence of the current?
It is; the moment the current stops, the weights fall away from the bar in obedience to the law of gravity.
How great weights have been lifted by magnets formed in this manner?
An electro-magnet constructed by Prof. Henry was capable of elevating and sustaining about a ton weight.
Upon what principle does the construction of the Morse magnetic telegraph depend?
Upon the principle that a current of electricity circulating about a bar of soft iron is capable of rendering it a magnet.
Why is it necessary, in conveying the telegraph wires, to support them upon glass or earthen cylinders?
These are used for the purpose of insuring the perfect insulation of the wires, since but for this the electricity would pass down a damp pole to the earth, and be lost.
Is there any truth in the idea that many persons have, that some principle passes along the telegraphic wires when intelligence is transmitted?
This supposition is wholly erroneous; the word current, as something flowing, conveys a false idea, but we have no other term to express electrical progression.
How can we gain an idea of what really takes place, and of the nature of the influence transmitted?
The earth and all matter are reservoirs of electricity; if we disturb this electricity at Boston by voltaic influence, its pulsations may be felt in Chicago. Suppose the telegraphic wire were a tube, extending from Boston to Chicago, filled with water. Now, if one drop more is forced into it at Boston, a drop must fall out at Chicago, but no drop was caused to pass from Boston to Chicago. Something similar to this occurs in the transmission of electricity.
What was the earliest important industrial application of electricity?
One of the earliest industrial applications of electricity was to the driving of street cars. The first electric railway was installed by Siemens, of Berlin, in 1882; and the system was quickly taken up and brought to a high state of development by American engineers. It is remarkable that the system of traction early adopted is the one generally used in America and Europe until the present date. It consists essentially of (a) a supply of continuous current at five hundred to five hundred and fifty volts, generated in (b) a central powerhouse, and transmitted to the car by means (c) of overhead conductors, whence by contact with a trolley wheel on a pole on the car it is led down to (d) two series-excited motors, which are placed electrically first in series with one another at starting, and then in parallel with one another when a sufficient speed has been attained.
To what well-known electrical machines did this give impetus?
Electric dynamos and motors. All such machines will convert the energy of mechanical motion into that of electricity in motion, or the reverse. The former conversion is done by dynamos, to which power is given by steam-engines or other such prime-movers, and made to generate in conducting circuits alternate or direct currents of electricity. Motors, on the other hand, receive the energy of electrical currents, either alternate or direct, and this produces motion of certain parts of the structure.
The theory of the action of a dynamo was first discovered by Faraday in 1831; it is intimately associated with that of a motor, for the principle of conservation of energy points out that either machine is reversible—that is to say, a dynamo may be used as a motor or a motor as a dynamo, though perhaps not so efficiently as when each fulfills the special function for which it was designed.
The Current in a Dynamo or Motor.—This brings us to the production of an electric current by the dynamo. In the dynamo we have a coil of wire moving across a magnetic field, alternately passing into this field and out of it. A magnetic field is produced, as we have just seen, by the steady movement of electrons, and we may picture it as being a region of the ether disturbed or strained by the effect of the moving electrons. When the coil of wire passes into the magnetic field, the electrons of its atoms are influenced powerfully and set in motion in one direction, so producing a current in the coil. As the coil passes away from the field, its electrons receive a second impetus, which checks their movement and starts them traveling in the opposite direction, and another current is produced. The coil moves continuously and regularly, passing into and out of the magnetic field without interruption; and so we get a current which reverses its direction at regular intervals, that is, an alternating current.
THE TELEGRAPH AND ITS WONDERFUL INSTRUMENTS
THE MORSE DIRECT INKING PRINTER
THE MORSE SOUNDER
| A | . _ | J | . _ _ _ | S | ... | 2 | .. _ _ _ |
| B | _... | K | _. _ | T | _ | 3 | ... _ _ |
| C | _. _. | L | . _.. | U | .. _ | 4 | .... _ |
| D | _.. | M | _ _ | V | ... _ | 5 | ..... |
| E | . | N | _. | W | . _ _ | 6 | _.... |
| F | .. _. | O | _ _ _ | X | _.. _ | 7 | _ _... |
| G | _ _. | P | . _ _. | Y | _. _ _ | 8 | _ _ _.. |
| H | .... | Q | _ _. _ | Z | _ _.. | 9 | _ _ _ _. |
| I | .. | R | . _. | 1 | . _ _ _ _ | 0 | _ _ _ _ _ |
THE MORSE TELEGRAPH CODE FOR LETTERS AND FIGURES
MODERN TELEGRAPHIC TYPEWRITING ATTACHMENT
A GOOD TYPE OF DYNAMO
What are some of the chief modern applications of electricity?
The field of applied electricity is one of the most extensive in modern science, invention and industry. Electricity in some form is now utilized in connection with lighting, telegraphy, the telephone, heating, motor boats, railways, aëroplanes, in metallurgy and the arts, clocks, bells and alarms, wireless telegraphy and telephony, submarine telegraphy, automobiles, cooking and domestic science, in medicine and in military science.
Give a brief account of wireless telegraphy.
In the case of ordinary telegraphy we always make use of extended metallic wires or conductors from the place from which the message is sent to the neighborhood to which it is desired to send it. In the case of wireless telegraphy no such conductors exist.
Among the most interesting of the many systems of wireless telegraphy now in vogue the modern Marconi, the De Forrest, the Fessenden, and the Poulsen are noteworthy. It is, however, with the name of Marconi that the introduction of wireless telegraphy will always be directly associated.
In 1888 Hertz had demonstrated in a remarkable series of experiments the existence of electro-magnetic waves, and had even shown how these might be produced, detected, and made to exhibit all the chief phenomena of wave-motion. Marconi’s great achievement lay in so controlling and regulating the dispatch and receipt of such waves as to make them record signals on a specially designed apparatus in accordance with the well-known Morse telegraphic system. His method, as first patented in 1896, was briefly as follows:
The Transmitter, by which the electromagnetic waves were generated and sent off into space in all directions, consisted of a battery connected through a key to the primary of an induction coil whose secondary terminals were joined to two brass balls between which there was a short air-gap. From one of these balls a wire was taken to earth, and from the other an aërial wire was led some distance up in the air. The closing of the primary circuit led to sparks passing across the air-gap, which produced electro-magnetic waves in the ether in exactly the same way as the dropping of a stone into a pool produces a series of concentric ripples.
The Coherer.—To receive and interpret these waves Marconi employed a “coherer” in circuit with a battery and having connection with an aërial wire on the one side and an earth wire on the other. The coherer consisted of a small glass tube not more than, say, two inches long by one-quarter inch in diameter, into the ends of which were fused two platinum wires leading to small metallic electrodes. These electrodes were brought quite near each other, and in the narrow gap between them was placed powdered metallic silver, antimony, etc. The resistance offered by this powder was so high, on account of small air-gaps between the particles, that no current could pass through.
Electro-magnetic waves, however, possess the peculiar property of breaking down the resistance of this powder whenever they impinge upon it. Hence as soon as a wave reached the coherer, the resistance practically vanished and a current passed round the circuit. It was a mere detail to arrange that this current should actuate a relay connected with a telegraphic instrument which would record the signal, and that a hammer would at the same time tap the coherer so as to agitate the powder and “decohere” it, setting up the resistance again for a fresh signal.
Improvements.—Since this system was devised many most important improvements have taken place. One of the most noticeable of these was Sir Oliver Lodge’s invention of tuning and syntonizing apparatus by which a transmitter and receiver are tuned to the same periodic oscillation, and thus a number of messages might be operated in the same field without interference. Lodge accomplished this to some extent by adding inductance coils and condensers to the circuits. Various other methods have been adopted to secure syntonization; but the resonance effects obtained are not great enough to make selective signaling certain.
The Generator.—In the modern Marconi system the energy for the transmitter is obtained from a generator working at one hundred and ten volts. The current is led through a key and an improved form of interruptor to the primary of the induction coil, whose secondary terminals communicate with the spark-gap. The spark-gap is in series with a condenser and the primary of a high tension transformer, of which latter one secondary terminal leads to the aërial and the other to the earth wire.
THE WIRELESS MESSAGE OVER LAND AND SEA
The Detector.—In the receptor the metallic coherer has been discarded for a magnetic detector. This instrument consists of a small glass tube through which travels an endless band of iron wires, moving round two grooved pulleys. Close to the tube are two permanent magnets, and round it is wound a primary coil consisting of one layer of wire. One end of this coil is led straight to earth; the other passes through a condenser to a tuning inductance coil leading in one direction to earth and in the other to the aërial. Above the primary coil on the glass tube a secondary coil is wound and connects with a telephone receiver. The action is simple. The electro-magnetic waves, reaching the aërial, set up oscillatory currents in the primary which act upon the magnetic field. Currents are thus generated in the secondary, which record the message in the telephone receiver by a series of taps corresponding to the Morse dashes and dots.
Courtesy of Marconi Wireless Co.
TELEPHONING FROM NEW YORK TO SAN FRANCISCO BY WIRELESS
The De Forrest system is very largely used in the United States, Japan, and elsewhere, and in its more recent modifications secures a high efficiency by means of a number of ingenious improvements.
Describe the wireless telephone.
As in wireless telegraphy, all modern systems of wireless telephony are based upon the action of electro-magnetic waves. It is impossible here to discuss all the various methods that have been devised, but the leading principles employed may be indicated, with special reference to some of the best-known systems. They may be classified according to the methods in which the waves are produced.
Spark Discharge Systems.—These rely for the generation of the Hertzian waves upon a spark discharge across an air-gap. The De Forrest system is perhaps the most popular of this type. In this system the spark discharge is utilized to produce waves of a frequency not less than one hundred thousand per second, the resulting sound being inaudible at the receiving station.
A microphone transmitter is employed with this apparatus. When the operator speaks into the transmitter, the variations of resistance act upon the waves in such a way as to produce a new series of waves of such frequency as to be audible at the receiver.
The receiving apparatus includes the usual antenna, and closed secondary circuit, comprising an inductance and a variable capacity, across the terminals of which an Audion delicate detector is introduced. This instrument depends upon the motions of the ions in a rarefied gas. It is one of the most sensitive detectors yet invented, and offers the great advantage of a practically total absence of time lag in recovery.
Singing-Arc Systems.—Duddell’s discovery of the singing arc in 1909 has been quickly followed by its application to radio-telephony and radio-telegraphy, first by Poulsen and subsequently by Fessenden, Stone, De Forrest, and others. Under certain conditions the electric arc can be made to emit a musical note, while at the same time it transforms a portion of the energy of its own direct current into oscillations. These are led into an oscillation circuit containing a condenser and inductance, and associated with [921] an antenna and earth line. The microphone transmitter may be included in a circuit associated with the inductance, in which case the voice acting on the resistance of the transmitter causes variations in the oscillating currents; or it may be associated with some part of the direct-current circuit, in which case it acts by affecting the current passing across the arc.
Any form of receiver may be used with this arc apparatus. The great advantage of this method is that the arc produces continuous oscillations of constant amplitude, and that the wave-length and frequency of the oscillations are subject to better regulation and control.
Advantages.—The advantages of wireless telephony over wireless telegraphy are many. One is that no skilled operator is required to translate the dot-and-dash signals; for in the latter one hears only long and short buzzes, whereas in the former one hears the actual spoken words. By means of wireless telephony the transmission of intelligence is far more direct and expeditious, and in times of emergency this not unfrequently becomes a very vital question indeed. An important characteristic of wireless telephonic communication is the exceptional clearness of the articulation, owing to the absence of the electrostatic capacity of metallic lines and cables which is always present in wire telephony.
Stronger currents, improved sending and receiving apparatus, and the application of new principles have now greatly extended the speaking range; and only recently distinct communication has been established by wireless telephony between New York and San Francisco. The use of the wireless telephone will be greatly extended, especially in naval, military, and shipping communication.