Electricity and Magnetism Part 3

The dynamo is a machine that converts mechanical into electrical energy, and the great practical value of energy in this form is that it can be distributed through a conductor economically for many miles. We can transmit mechanical power by means of a rope or cable for a limited distance, but at tremendous loss through friction. We can transmit power through pipes by compressed air or steam, but there is a great loss, especially in the case of steam, by condensation from cold. None of these methods are available for long distances. Another advantage electricity has over other forms of energy is the speed with which it can be transmitted from one place to another. In this respect it has no rival except light. But we have not been able to harness light and make it available to carry either freight or news, except in the latter case for a short distance by flashing it in agreed signals.

The heliostat can be used when the sun shines to transmit news by flashes of sunlight chopped up into the Morse code and thrown from point to point by a moving mirror. But this is limited as to distance; besides, the sun does not always shine. It has the disadvantage in that respect that the old semaph.o.r.e-telegraph did that was in use in Wellington's day. These semaph.o.r.es were constructed in various ways, but a common form was that of moving arms that could be seen from hill to hill or point to point. By a code of moving signals news was repeated from point to point and it can be easily imagined that many mistakes occurred, to say nothing of the time it required for repet.i.tion. When the battle of Waterloo was fought--so the story goes--news was sent to England by means of the semaph.o.r.e-telegraph. The dispatch read, "Wellington defeated--" At that point in the message a thick fog came up and lasted for three days, so that no further news could be sent or received. In the telegraphic parlance of to-day the line was "busted."

For three long days all London was in deep mourning, when finally the fog lifted, which repaired the telegraphic line, and the balance of the dispatch was received--"the French at Waterloo." Mourning changed to rejoicing and the English have rejoiced ever since when they think of either Wellington or Waterloo.

But to return to the dynamo. The name dynamo is an abbreviation for dynamo-electric machine. A machine for producing dynamic electricity.

There are many forms of the dynamo, just as there are in the evolution of every important machine, and there will be many more. But the fundamental, underlying principle of them all is contained in an experiment made by Faraday. Faraday took the soft iron "keeper" of a permanent magnet and wound insulated wire around it and brought the two ends of the wire close together. He now placed the keeper, with the wire wound around it, across the poles of the permanent magnet, and wrenched it away suddenly, when he observed a spark pa.s.s between the ends of the wires. This would occur when he approached the poles as well as when he took it away. He discovered that the currents were momentary and occurred at the moment of approach or recession, and that the currents developed by the approach were of opposite polarity to those occurring at the recession. When the "keeper" was put on the poles of the magnet it was magnetized by having its molecular rings broken up and the poles of the little natural magnets all turned in one direction.

During the time that the molecules of the keeper are changing they are in a dynamic or moving condition. By some mysterious action of the ether between the iron and the wire wrapped around it there is a corresponding molecular action in the wire that is dynamic for a moment only, and during that moment we have the phenomenon of an electric current. When the magnet and soft iron are separated this molecular state of strain is relieved and the molecules of both the iron and the wire wound about it return to normal, and in the act of returning we have a dynamic or moving condition, resulting in a current, only in the opposite direction. (See Chap. VI.)

Now mount the permanent magnet in a frame and mount the soft iron with the wire on it (which in this shape is an electromagnet) on a revolving arm and so set it on the arm that its ends will come close to, but not touch, the poles of the permanent magnet. Now revolve the arm, and every time the electromagnet or keeper approaches the permanent magnet a current of one polarity will be momentarily developed in the wire of the electromagnet, which is moving. When it is opposite the poles, it has reached the maximum charge and, now, as it pa.s.ses on it discharges and a current of the opposite polarity is developed in the wire. The more rapidly we revolve the arm the more voltage (electrical pressure) the current it develops will have.

It will be plain to all that we might make the electromagnet stationary and revolve the permanent magnet and get the same result. If the permanent magnet were strong enough and the electromagnet the right size as to iron, windings, etc., and we revolve the arm with sufficient rapidity, we could get an alternating current of electricity that would produce an electric light. I have not and cannot here give you the construction of a modern alternating-current dynamo. I have simply described the simplest form of dynamo, and all of them operate upon the fundamental principle of a permanent magnetic field and an electromagnet, moving in a certain relation to each other. The field may revolve or the electromagnet may revolve, whichever is the most convenient to construct. The field-magnet may be a permanent magnet or an electromagnet, made permanent during the operation of the dynamo by a part of the current generated by the machine being directed through a coil surrounding soft iron; or the field-current may come from an outside source. This is the kind of field-magnet universally used for dynamo work, as a much stronger magnetism is developed in this way than it is possible to obtain from any system of permanent steel magnets.

The usual construction is to have a stationary field-magnet and then a series of electromagnets mounted and revolving upon a shaft in the center of the magnetic field. The rotating part is called the armature, and is so wound with insulated wire that successive induced currents are created in the armature windings and discharged through brushes which rest on revolving segments that connect with the armature windings.

These induced currents succeed each other with such rapidity as to amount in practice to a steady current. However, the separate pulsations are easily heard in any telephone when the circuit is near to that of a dynamo circuit. The dynamo current is not nearly so steady as the battery current, although both are probably made up of separate discharges. In the dynamo there is a discharge every time the electromagnet of the armature cuts through the lines of force of the magnetic field, and in the galvanic battery every time a molecule is broken up and its little measure of energy is set free. In the dynamo the pulsations are so far apart as to make a musical tone of not very high pitch, but in the galvanic battery the pitch of the tone, if there is one, would require a special ear to hear it--one tuned, it may be, up near the rate of light vibration.

There are two types of dynamo, one generating a direct and the other an alternating current. (By alternating we mean first a positive and then a negative current impulse.) We cannot enter into a technical description of the dynamo in a popular treatise such as this.

The dynamo has evolved from the germ discovered by Faraday, till to-day it is a machine, the construction of which requires the highest cla.s.s of engineering skill. When in action it seems like a great living presence, scattering its energy in every direction in a way that is at once a marvel and a blessing to mankind. But we must not give all the credit to the dynamo. As the moon shines with a reflected light, so the dynamo gives off energy by a power delegated to it by the steam-engine that rotates it, and the steam-engine owes its life to the burning coal, and the burning coal is only giving up an energy that was stored ages ago by the magic of the sunbeam; and the sun--? Well, we are getting close on to the borders of theology, and being only scientists we had better stop with the sun.

There is still another way of generating electricity besides those that we have named; which are friction, chemical action, and the magneto-electric mode of generating a current. Electricity may be generated by heat. If we connect antimony and bis.m.u.th bars together and apply heat at the junction of the metals and then connect the free ends of the two bars to a galvanometer, it will indicate a current. These pairs can be multiplied, and in this way increase the voltage or pressure, and, of course, increase the current, if we a.s.sume that there is resistance in the circuit to be overcome. If there were absolutely no resistance in the circuit--a condition we never find--there would be no advantage in adding on elements in series.

Substances differ in their resistance to the pa.s.sage of electricity--the less the resistance the better the conductor. The German electrician, G.

S. Ohm (1789-1854), investigated this and propounded a law upon which the unit for resistances is based, and this unit takes his name and is called the "ohm."

Any two metals having a difference of potential will give the phenomena of thermo-electricity. Antimony and bis.m.u.th having a great difference of potential are commonly used. The use made of thermal currents is chiefly for determining slight differences of temperature. An apparatus called the thermo-electric pile has been constructed out of a great number of pairs of antimony and bis.m.u.th bars. This instrument in connection with a galvanometer makes a most delicate means of determining slight changes of temperature. If one face of a thermopile is exposed to a temperature greater than its own, the needle will move in one direction; if to a temperature lower than its own, the needle will be deflected in the opposite direction. If both faces of the pile are exposed to the same changes of temperature simultaneously, of course no electrical manifestations will occur.

The earth is undoubtedly a great thermal battery that is kept in action by the constant changes of temperature going on at the earth's surface, caused by its rotation every twenty-four hours on its axis. The sun, of course, is at some point heating the earth, which at other points is cooling, making a constant change of potential between different points.

If we heat a metal ring at one point a current of electricity will flow around it--especially if it is made of two dissimilar metals--until the heat is equally distributed throughout the ring.

Some years ago, when the Postal Telegraph Company first began operations between New York and Chicago, the writer made observations twice a day for some time of the temperature and direction of the earth-current. The first two wires constructed gave only two ohms resistance to the mile, which facilitated the experiments. I found that in almost every instance the current flowed from the point of higher temperature to the lower. If the temperature in New York were higher at the time of observations than in Chicago the current would flow westward, and if the conditions were reversed the current would be reversed also.

CHAPTER VIII.

ATMOSPHERIC ELECTRICITY.

Nature has another mode of generating electricity, called atmospheric.

The normal conditions of potential between the earth and the upper atmosphere seem to be that the atmosphere is positively electrified and the earth negatively. These conditions change, apparently from local causes, for short periods during storms. In some way the sun's rays have the power directly or indirectly to give the globules of moisture in the air a potential different from that of the earth.

In clear weather we find the air near to the earth in a neutral condition, but gradually a.s.suming the condition of a positive charge as we ascend; so that the upper air and the earth are oppositely charged like the two sides of a Leyden jar or two leaves of a condenser. This condition is intensified and localized when a thunder-cloud pa.s.ses over the earth. The moisture globules have been charged with potential energy by the power of the sun's rays when evaporation took place; but in this state the energy is neither heat nor electricity, but a state of strain like a bent bow or a wound-up spring. When these moisture globules condense into drops of water the potential energy is set free and becomes active either as heat or electricity. The cloud gathers up the energy into a condensed form, and when the tension gets too great a discharge takes place between the cloud and the earth or from one cloud to another, which to an extent equalizes the energy.

In most cases of thunder and lightning it is only a discharge from cloud to cloud unequally charged. This does not relieve the tension between the earth and the cloud, but distributes it over a larger area. The reason for this constant electrical difference between the earth and the upper regions of atmosphere is not well understood, except that primarily it is an effect of the sun's rays. Evaporation may and probably does play a part, and the same causes that give rise to the auroral display may contribute in some way to the same result.

Evaporation does not always take place at the earth's surface. Cloud formations may be evaporated in the upper air into invisible moisture spherules, and charged at the time with potential energy. If we go up into a high mountain when the conditions are right, we can witness the effect of this condition of electrical charge or strain between the upper regions of the atmosphere and the earth, and the tendency to equalize the potentials between the clouds and the earth. Often one's hair will stand on end, not from fright, but from electricity pa.s.sing down from the upper regions to the earth. When the tension is very great a loud hissing sound as of many musical tones of not very good quality may be heard, and a brush or fine-pointed radiation of electricity may be seen from every point, even from your finger-ends. The thunder is not usually so loud on high mountains for two reasons--one because the air is rare, but the chief reason is that the mountain acts as a great lightning-rod and gradually discharges the cloud or atmosphere, for often the phenomena may occur when the sky is clear.

I remember being on top of what is called the Mosquito Range, between Alma and Leadville in Colorado, during the pa.s.sage of a thunder shower.

There was no heavy thunder, but a constant fusillade of snapping sounds, accompanied by flashes not very intense. I could feel the shocks, but not painfully. A part of the time I was in the cloud and became for the time being a veritable lightning-rod. After the cloud pa.s.sed it crawled down the mountainside as if clinging to it, all the time bombarding it with little electric missiles. After the cloud left the mountain and pa.s.sed over the valley I could hear loud thunder, because the charge would have to acc.u.mulate quite a quant.i.ty, so to speak, before it could discharge. These heavy discharges when the cloud is some distance from the earth would be dangerous to life, while the light ones, when the cloud is in contact with the earth, are not.

Many wonderful and destructive effects come from these lightning discharges and many lives are lost every year from this cause. I do not suppose it is possible to be on one's guard continually, but many lives are needlessly lost either from ignorance or carelessness. Although there is a just prejudice against lightning-rods as ordinarily constructed, it is still just as possible to protect your house and its inmates from the destroying effects of lightning as from rain. If, for instance, we lived in metal houses that had perfect contact all round them with moist earth, or better, with a water-pipe that has a large surface contact with the earth, the lightning would never hurt the house or the inmates. In such a case you simply carry the surface of the earth to the top of your house, electrically speaking, and neutralization takes place there in case the lightning strikes the house. A house that is heated with hot water can easily be made lightning-proof by a little work at the top and bottom of the heating system. All the heavy metal of the house should be a part of the lightning-rod. Points should be erected at the chimneys, and if there is a metal roof they should be connected with it. Then connect the roof with rods from several points with the ground. Here is where most rods fail. The ground connection is not sufficient. The earth is a poor conductor, and we have to make up by having a large metal surface in contact with it. It is best to have the rod connected with the water pipe, if there is one, and have it connected with metal running all around the house as low down as the bottom of the cellar, for sometimes there is an upward stroke, and you never can tell where it is coming up. If you have a heating system it should be thoroughly grounded and the top pipe connected with the rods at the chimneys. These rods need not be insulated as is the usual practice.

If you are outdoors during a thunder-storm never get under a tree, but if you are twenty or thirty feet away it may save your life, because, if it comes near enough to strike you, it will probably take the tree in preference. It seeks the earth by the easiest pa.s.sage. An oil-tank and a barn are dangerous places, if the one has oil in it and the other is filled with hay and grain. A column of gas is rising that acts as a conductor for lightning. Of course if the barn is properly protected with rods it will be safe. Sometimes a cloud is so heavily charged that the lightning comes down like an avalanche, and in such a case the rods must have great capacity and be close together to fully protect a building.

There is a popular notion that rods draw the lightning and increase the damage rather than otherwise. This is a mistake. Points will draw off electricity from a charged body silently. It would be possible to so protect a district of any size in such a way that thunder would never be heard within its boundaries if we should erect rods enough and run them high enough into the upper air. The points--if they were close enough together--would silently draw off the electricity from a cloud as fast as it formed, and thus effectually prevent any disruptive discharge from taking place.

CHAPTER IX.

ELECTRICAL MEASUREMENT.

Having given a short account of some of the sources of electricity, let us now proceed to describe some of the practical uses to which it is put, and at the same time describe the operation of the appliances used.

Before proceeding further, however, we ought to tell how electricity is measured. We have pounds for weight, feet and inches for lineal measure, and pints, quarts, gallons, pecks and bushels for liquid and dry measure, and we also have ohms, volts, amperes and ampere-hours for electricity.

When a current of electricity flows through a conductor the conductor resists its flow more or less according to the quality and size of the conductor. Silver and copper are good conductors. Silver is better than copper. Calling silver 100, copper will be only 73. If we have a mile of silver wire and a mile of iron wire and want the iron wire to carry as much electricity as the silver and have the same battery for both, we will have to make the iron wire over seven times as large. That is, the area of a cross-section of the iron wire must be over seven times that of the silver wire. But if we want to keep both wires the same size and still force the same amount of current through each we must increase the pressure of the battery connected with the iron wire. We measure this pressure by a unit called the "volt," named for Volta, the inventor or discoverer of the voltaic battery. The volt is the unit of pressure or electromotive force. (In all these cases a "unit" is a certain amount or quant.i.ty--as of resistance, electromotive force, etc.--fixed upon as a standard for measuring other amounts of the same kind.)

The iron wire offers a resistance that is about seven times greater than silver to the pa.s.sage of the current. To ill.u.s.trate by water pressure: If we should have two columns of water, and a hole at the bottom of each column, one of them seven times larger than the other, the water would run out much faster from the larger hole if the columns were the same height. Now, if we keep the column with the larger hole at a fixed height a certain amount of water will flow through per second. If we raise the height of the column having the small hole we shall reach a point after a time when there will be as much water flow through the small hole per second as there is flowing through the large hole. This result has been accomplished by increasing the pressure. So, we can accomplish a similar result in pa.s.sing electricity through an iron wire at the same rate it flows through a silver wire of the same size, by increasing the pressure, or electromotive power; and this is called increasing the voltage.

The quality of the iron wire that prevents the same amount of current from flowing through it as the silver is called its resistance. The unit of resistance, as mentioned in the last chapter, is called the ohm, and the more ohms there are in a wire as compared with another, the more volts we have to put into the battery to get the same current.

The unit for measuring the current is called the "ampere," named after the French electrician, A. M. Ampere (1789-1836).

Now, to make practical application of these units. The volt is the potential or pressure of one cell of battery called a standard cell, made in a certain way. The electromotive force of one cell of a Daniell battery is about one volt. One ohm is the resistance offered to the pa.s.sage of a current having one volt pressure by a column of mercury one millimeter in cross-section and 106.3 centimeters in length. Ordinary iron telegraph-wire measures about thirteen ohms to the mile. Now connect our standard cell--one volt--through one ohm resistance and we have a current of one ampere. Unit electromotive force (volt) through unit resistance (ohm) gives unit of current (ampere). It is not the intention to treat the subject mathematically, but I will give you a simple formula for finding the amount of current if you know the resistance and the voltage. The electromotive force divided by the resistance gives the current. C = E/R or current (amperes) equals electromotive force (volts) divided by the resistance (ohms).

But still further: One ampere of current having one volt pressure will develop one watt of power, which is equal to 1/746 of a horse-power.

(The watt is named in honor of James Watt, the Scottish inventor of the steam-engine--1786-1813). In other words, 746 watts equal one horse-power. By multiplying volts and amperes together we get watts.

If we want to carry only a small current for a long distance we do not need to use large cells, but many of them. We increase the pressure or voltage by increasing the number of cells set up in series. If we have a wire of given length and resistance and find we need 100 volts to force the right amount or strength of current through it, and the electromotive force of the cells we are using is one volt each, it will require 100 cells. If we have a battery that has an E. M. F. of two volts to the cell, as the storage-battery has, fifty cells would answer. If we want a very strong current of great volume, so to speak, for electric light or power, and use a galvanic battery, we should have to have cells of large surface and lower resistance both inside and outside the cells.

When dynamos are used they are so constructed that a given number of revolutions per minute will give the right voltage. In fact, the dynamo has to be built for the amount of current that must be delivered through a given resistance. The same holds good for a dynamo as for a galvanic battery. If any one factor is fixed, we must adapt the others to that one in order to get the result we want. There are many other units, but to introduce them here would only confuse the reader. The advanced student is referred to the text-books.

With this much as a preliminary we are prepared to take up the applications of electricity, which to most people will be more interesting than what has gone before.

CHAPTER X.

THE ELECTRIC TELEGRAPH.

In the year 1617 Strada, an Italian Jesuit, proposed to telegraph news without wires by means of two sympathetic needles made of loadstone so balanced that when one was turned the other would turn with it. Each needle was to have a dial with the letters on it. This would have been very nice if it had only worked, but it was not based on any known law of nature.

Many attempts at telegraphing with electricity were made by different people during the eighteenth century. About 1748 Franklin succeeded in firing spirits by means of a wire across the Schuylkill River, using, as all the other experimenters had done, frictional electricity. In 1753 an anonymous letter was written to Scott's Magazine describing a method by which it was possible to communicate at a distance by electricity. The writer proposed the use of a wire for each letter of the alphabet, that should terminate in pith b.a.l.l.s at the receiving end, and under the b.a.l.l.s were to be strips of paper corresponding to the letters of the alphabet. The message was to be sent by discharging static electricity through the wire corresponding to the first letter of a word when the paper would be attracted to the pith ball and read by the observer. Then the wire corresponding to the second letter of the word was to be charged in like manner, and so on till the whole message was spelled out. This was the first practical (i.e., possible) suggestion for a telegraph. The writer also proposed to have the wires strung on insulators, which was a great advance over the other attempts.

The communication was anonymous, as no doubt, like many others, the author feared the ridicule of his neighbors. It requires a vast amount of moral courage to stand up before the world and openly advocate some new theory that has never come within the experience of any one before.