Current (electricity)

“ In the phenomenon called the electric current the constant passage of electricity through the medium tends to restore the state of polarization as fast as the conductivity of the medium allows it to decay. Thus the external agency which maintains the current is always doing work in restoring the polarization of the medium, which is continually becoming relaxed, and the potential energy of this polarization is continually becoming transformed into heat, so that the final result of the energy expended in maintaining the current is to raise the temperature of the conductor. ”

“ When an ether-wave is set up by a discharge of electricity into the air it strikes the perpendicular wire of the receiver, and that portion of the wave that strikes is converted into electricity, which is called an induced current. It is this current, as it discharges through the coherer to the earth, that causes the filings to unite so as to close the local circuit and operate the sounder. To send a message it is only necessary to make the discharges into the air, at the sending-end, correspond to the Morse alphabet. ”

“ So, if we bring the two carbons in an arc-lamp together they really form part of the [72] wire, and do not interrupt the current in its travels, but, if we separate the carbons, we make a gap which the current must jump across if it wants to go on. As the volts, or pressure, is so great, the current must jump, and this against the resistance or opposition in an arc-lamp is that which gives the current so much work to do. ”

“ In the case of the electric current, we find that, when the electromotive force begins to act, it does not at once produce the full current, but that the current rises gradually. What is the electromotive force doing during the time that the opposing resistance is not able to balance it? It is increasing the electric current.
Now an ordinary force, acting on a body in the direction of its motion, increases its momentum, and communicates to it kinetic energy, or the power of doing work on account of its motion. ”

“ The alternating-current dynamo gives a surging of electricity back and forth in the wires, and this may be distinguished from the direct current by its feeling; when, however, the number of alternations per second is increased very greatly, one may receive through the body considerable quantities of electricity without feeling it. With a very high frequency current one may put himself in circuit and light a 16-candle-power lamp without any disagreeable sensation. ”

“ We have just the same effects in electricity. If we place some substance which is practically a non-conductor, or insulator, such as rubber, around an electric wire, or in the path of an electric current, the electricity, acted upon by the volts of pressure, cannot escape, because the insulation keeps it from doing so, just as the iron of the pipe keeps the [7] water from escaping. Thus, you see, the volt does not itself represent electricity, but only the pressure which forces it through the wire. ”

“ If the discharge from such a Leyden jar or a condenser is passed through a coil of wire acting as a primary, and the primary is provided with a secondary coil containing a larger number of turns, the secondary will produce a peculiar current known as high-frequency electricity. High-frequency currents reverse their direction of flow or alternate from one hundred thousand to one million times a second. ”

“ No current at all flows save at the instant of making or breaking the current. In this respect, we are reminded of the behavior of the soft iron within the coil, which gives origin to a current of electricity when it is made to approach a magnet or recede from it, but gives no current so long as it is still. ”

“ It is greater, for instance, in dense air than in rare air, and greater in glass than in air, but in every case, if the electromotive force be made great enough, the dielectric gives way and its insulating power is destroyed, so that a current of electricity takes place through it. It is for this reason that distributions of electricity for which the electric resultant force becomes anywhere infinite cannot exist in nature. ”

“ When the alternations of the current are very rapid, the ohmic resistance ceases to play a dominant part, and the ordinary equations connecting electromotive force, induction, and current are equivalent to the conditions that the currents shall be so distributed as to make the electrokinetic or magnetic energy a minimum. Consider now the case of a single straight wire of circular cross-section. ”

“ We cannot pick up positive units of electricity from the ground as if they were pebbles and carry them by hand to the sphere; we must proceed in a different way. Let us then take a frictional machine and connect its negative wire to the ground, and the positive to the sphere. If the machine is worked, it will push negative electricity into the ground and positive on to the sphere. In other words, a charging current will flow along that wire, and more and more electricity will accumulate on the sphere the longer the machine is at work. ”

“ The best conductors offer some resistance to the passage of the current and the best insulators conduct to some extent. If we make a comparison of electric conductors we find that the metals that conduct heat best also conduct electricity best. This, it seems to me, is a confirmation of the atomic theory of electricity so far as it means anything. If a good conductor, as silver, is subjected to intense cold by putting it into liquid air, its conductivity is greatly increased. It is well known that [Pg 48] heating a conductor ordinarily diminishes its power to conduct electricity. ”

“ We have seen that when too large a current passes through a wire, the wire becomes heated and may even be melted. Buildings are wired to use certain currents, and if from any cause much more current than the regular amount should suddenly pass through the service wires into the house, [120] the various smaller wires would become overheated, and perhaps melt or start a fire. An accidental short circuit, for example, would so reduce resistance that too much current would suddenly rush through the wires. ”

“ This transformer, as heretofore explained, does not change the voltage of the current, but simply transforms what was an alternating into a direct current. By alternating current we mean one that is made up of impulses of alternating polarity—first a positive and then a negative. The direct current is one whose impulses are all of one polarity. The direct current is required for all purposes where electrolysis (chemical decomposition by electricity, as of silver for silver-plating, etc.) is a part of the process. ”

“ If now the primary current is stopped, a secondary current is observed, which is in the same direction as the primary current. Every variation of the primary current produces electromotive force in the secondary circuit. When the primary current increases, the electromotive force is in the opposite direction to the current. When it diminishes, the electromotive force is in the same direction as the current. When the primary current is constant, there is no electromotive force.
These effects of induction are increased by bringing the two wires nearer together. ”

“ Surging Character of High-tension Currents.—When a current of electricity is sent through a wire, hundreds of miles in length, the current surges back and forth on the wire many thousands of times a second. Light comes to us from the sun, over 90,000,000 of miles, through the ether. ”

“ If a single wire be moved like the spoke of a wheel near a magnetic pole, a current of electricity is determined through it from one end towards the other. If a wheel be imagined, constructed of a great number of these radii, and this revolved near the pole, in the manner of the copper disc (85.) , each radius will have a current produced in it as it passes by the pole. ”

“ It seems to pile up at points and corners, and we say that it is denser at such places than at well-rounded parts of a charged conductor. All points and sharp places should be removed from a conductor, if it is desired to keep a charge for any length of time.
Electrification may escape from a point so rapidly that currents are produced in the surrounding air. As the particles of air become charged, they repel each other. The movement of the air particles may be so great that a lighted candle will be affected when placed near the point. This current of air is called electric wind. ”

“ How quickly a spoon inserted into a hot cup of tea becomes uncomfortably hot, though the hand can be held very close to the liquid without feeling more than a gentle warmth. And we all have noticed that the very least air-gap in an electric circuit effectively breaks a current capable of traversing miles of wire. If the current is so intense that it insists on passing the gap, it leaps across with a report, making a spark that is at once intensely bright and hot. Metal wires are to electricity what speaking tubes are to sound; they are as it were electrical tubes through the air and ether. ”

“ In the arc-lamp the electric circuit is broken at the point where light is required. In glow or incandescent lamps the current is only hindered by the interposition of a bad conductor of electricity, which must also be incombustible. Just as a current of water flows in less volume as the bore of a pipe is reduced, and requires that greater pressure shall be exerted to force a constant amount through the pipe, so is an electric current choked by its conductor being reduced in size or altered in nature. ”

“ We do not know precisely what electricity is, and by implication we do not know precisely what an electric current is, but we have all requisite knowledge about it for practical purposes. An electric current is a something which occurs in a conductor, i.e. a piece of metal when it connects two points between which there is electric pressure. The current may be only momentary, as in the case of Franklin's kite with a wire attached which was sent up into a thundercloud, or when a so-called Leyden jar is discharged ”

“ If the two wires attached to the plates are now connected with electric lamps, or a motor, or other device, the active material will develop energy in the effort to again change its nature. This energy takes the form of an electric current, which leaves the battery and passes through the conductors and operates the lamps, motors, or other devices in its passage. ”

“ These generators were fitted with a spinning switch or commutator (one of the neatest gadgets Edison ever invented) to make the current flow in unidirectional pulses (D.C.) In 1876 all electrical equipment was powered by direct current.
Today mains electricity plays a vital part in our everyday lives and its applications are widespread and staggering in their immensity. But we must not forget that popular demand for this convenient form of power arose only about 100 years ago, mainly for illumination. ”

“ At a certain distance from the point the resultant force is not sufficient to break through the insulation of the air, so that the electric current is checked, and the electricity accumulates in the air round the point. ”

“ A greater amount can be carried through a small conductor; thus greatly reducing the cost of an electrical plant that distributes power to a distance. A transformer is an apparatus that changes the current from one voltage to another.
In the ordinary electric-light plant, such as is used in a small town or village, the current that is sent out from the power-station has a pressure of from 1000 to 1500 volts, according to the distance to which it is sent. ”

“ Among the startling facts brought out in these experiments is that although a current of electricity, either direct or alternating, from ordinary dynamos under fifteen hundred or two thousand volts electro-motive 'force will kill, yet under alternations of a million to a million and a half per second a voltage of fifty thousand produces no shock or injury. ”

“ Fig. 117. Arc-Lighting Circuit.
A high voltage is necessary in order to cause the current to leap across the gap made by the separation of the electrodes
p. 166
The Incandescent System.—This method is entirely different from the arc system. It has been stated that certain metals conduct electricity with greater facility than others, and some have higher resistance than others. If a certain amount of electricity is forced through some metals, they will become heated. ”

“ For instance, take the current that is sent to Buffalo, a distance of from twenty to thirty miles. The current first runs to a transformer connected with the power-house, where it is "stepped-up" (to use the parlance of the craft) from a voltage of 2200 to 10,000. ”

“ The increased length has added resistance, and, as you see by the ammeter, cut the current down from 8 to 7.5 amperes. I will now darken the room and you find that it is giving light enough to read by. But you notice that the light is growing dimmer, its colour is growing [133] redder, and the ammeter indicates that less current is passing. I will cut off the current and let you examine the wire and you notice that a crust has formed upon it. This is due to the oxygen of the air which unites with the iron, forming iron rust. Iron rust does not conduct electricity. ”

“ One grain of water, acidulated to facilitate conduction, will require an electric current to be continued for three minutes and three quarters of time to effect its decomposition, which current must be powerful enough to retain a platina wire 1/104 of an inch in thickness183, red-hot, in the air during the whole time; and if interrupted anywhere by charcoal points, will produce a very brilliant and constant star of light. If attention be paid to the instantaneous discharge of electricity of tension, as illustrated in the beautiful experiments of Mr. ”