Popular Science Monthly/Volume 26/November 1884/What Is Electricity?
|WHAT IS ELECTRICITY?|||
THE conjunction of the meeting of the American Association with the opening of the Electrical Exposition and the sittings of the National Electrical Congress leads me to say a few words upon a question which we all ask ourselves, and to which we have hitherto had no response: "What is electricity?"
After I have concluded, you will probably still ask yourselves, "What is electricity?" All I can hope to do is to make you ask yourselves the question with more humility, and a greater consciousness of ignorance; for the ignorant man, I have found, is generally sure that he knows what electricity is; and, the more learned a person is, the more he is convinced that he does not know what electricity is.
There is an advantage in sounding the depths of our ignorance, and in surveying, even from a small Mount Pisgah, the paths we have traversed, and the great promised land which lies before us. In the beginning I must express my conviction that we shall never know what electricity is, any more than we shall know what energy is. What we shall be able probably to discover is, the relationship between electricity, magnetism, light, heat, gravitation, and the attracting force which manifests itself in chemical changes. We have one great guiding principle which, like the pillar of cloud by day, and the pillar of fire by night, will conduct us, as Moses and the Israelites were once conducted, to an eminence from which we can survey the promised scientific future. That principle is the conservation of energy. To-day we see clearly that there are not different kinds of forces; that light is not one thing and heat another; that, in truth, we should blot the word light from our physical text-books ; that electricity and magnetism have their equivalents in heat, and heat in mechanical work. The ancients had a god for every great manifestation of Nature—a god of peace, a god of war, a god of the land, a god of the sea. Fifty years ago scientific men were like the ancients. There was a force attached to every phenomenon of Nature. Thus, there were the forces of electricity and magnetism, the vital forces, and the chemical forces. Now we accept treatises on mechanics which have the one word "Dynamik" for a title; and we look for a treatise on physics, which shall be entitled "Mechanical Philosophy," in which all the phenomena of radiant energy, together with the phenomena of energy, which we entitle electricity and magnetism, shall be discussed from the point of view of mechanics. It is true that Mascart and Joubert have entitled their treatise on electricity and magnetism "The Mechanical Theory of Electricity and Magnetism"; but what we are to have in the future is a treatise which will show the mechanical relations of gravitation, of so-called chemical attracting force and electrical attracting force, and the manifestations of what we call radiant energy.
When we survey the field of modern physics, we see that there is a marked tendency to simplify our conceptions. The question is sometimes asked. How shall the man of the future be able to make any advance, since it now takes one until middle age to gain familiarity with the vortex theories, with quaternions, and the more or less complicated mathematical analysis which characterizes our mechanical theory of electricity to-day? It is evident that much of our complicated scaffolding is to be taken down, and the student of electricity in the future will start with, perhaps, the laws of vortices as axioms, just as the student in physics to-day starts with the truth that the energy which we receive from the sun does not exist either as light or heat in the space between us and the sun, but may be electro-magnetic, or even in an unsuspected form; and that light and heat are merely manifestations of waves of energy differing only in length.
We have reduced our knowledge of electricity and magnetism to what may be called a mechanical system, so that, in a large number of cases, we can calculate beforehand what will take place, and we are under no necessity of trying actual experiments.
Thus, a portion of our knowledge of electro-magnetism is very much in the condition of our knowledge of what may be called geometrical astronomy, in distinction to physical astronomy, "We can calculate what will take place with small errors, which arise merely from the faults of observation, and not from a want of knowledge of conditions, or from the errors of a defective theory. It is probable, for instance, that the correct form of a dynamo -machine for producing the electric light can be calculated, and the plans drawn with as much certainty as the diagrams of a steam-engine are constructed. There is a department of electricity corresponding, perhaps, to hydraulics, in which the electrical engineer can find full employment in subjecting perfectly definite conditions to exact calculation. We can congratulate ourselves, therefore, in having a large amount of systematic knowledge in electricity, and we see clearly how to increase this systematic knowledge; for we have discovered that a man, to become an electrician, can not expect to master the subject of electricity, who has not made himself familiar with thermo-dynamics, with analytical mechanics, and with all the topics now embraced under the comprehensive title of physics.
Some may think that an electrician is a narrow specialist. I can only invite such persons to engage in the study of "What is electricity?"
In standing upon our scientific Mount Pisgah, we can survey the beaten roads by which we have advanced, and can see partially what has been good and what has been bad in the theories which have stood in the place of the leaders of the Israelites and have conducted us thus far. Out of all the theories—the two-fluid theory, the one-fluid, or Franklin theory, and the various molecular theories—not one remains to-day under the guidance of which we are ready to march onward. The two-fluid theory serves merely to fix the ideas of the student whose mind is new to the subject of electricity, I think I can safely affirm that no scientific man of the present believes that there is even one electric fluid, to say nothing of the existence of two. We have discovered that we can not speak of the velocity of electricity. We do not know whether the rate of propagation of what we call an electrical impulse is infinitely slow or infinitely fast. We do not know whether what we call the electrical current in a conductor is due to molecular motions infinitely faster than those of outlying molecules, or whether there is a sudden comparative cessation of molecular motion in the wire through which the current manifests itself, compared with the molecular motions outside the wire, for this might produce the electrical phenomena we observe. We do not know whether any molecular motion is concerned in the manifestation of energy which we call electrical. All that we can truly say is, we have a healthy distrust of our theories, and an abiding faith in that pillar of cloud by day and the pillar of fire by night—the doctrine of the conservation of energy.
Having thus outlined the present condition of our knowledge, and of our comprehension of the bearing and tendencies of physical science, let us strive, with the most powerful instruments we have, to survey the promised land which is undoubtedly to be the possession of those who come after us. It is one thing to become familiar with all the applications of the mechanical theory of electricity, and another to make an advance in the subject so that we can see the relations of electrical and magnetic attraction to the attraction of gravitation and to what we call chemical attraction. To this possible relationship I wish to call your attention to-day.
I am forced to believe that the new advances in our knowledge of electrical manifestations are to come from a true conception of the universality of electrical manifestations, and from the advance in the study of molecular physics. Picture to yourselves the position of an investigator in this world. A person on the moon could only conceive of this audience as a molecule made up of many atoms. He could not measure the energy you manifest by moving about—the heat energy—the electrical energy due to the friction of your envelopes. Indeed, he could only suppose your existence, just as we imagine the existence of a molecule of a crystal. Now, the distances we force molecules apart by many of our chemical processes seem extremely small to us; but how immense they really are compared with the distances apart of the atoms! Is it not as if we should take a stone from the moon or from Venus and place it upon the earth in the time of one second? You can imagine, from the familiar spectacle of a meteor, the heat and the electricity that would result. Yet, in respect to relative distances, do we not do something similar when we break a crystal or pour acid upon a metal, or strike a dynamite-cartridge? We are infinitely small ourselves compared with the great universe about us; yet our task is to comprehend the motions of aggregations of atoms infinitely smaller than that aggregation which we call man.
When we break a crystal mechanically, we have a development of electricity. When we heat certain crystals—tourmaline, for example—besides the strain among the molecules of the crystal which is produced by the increased rates of vibration, we have a difference of electrical potential. When we let an acid fall from the surface of a metal, the metal takes one state of electrification and the drop of acid the other—in other words, we produce a difference of electrical potential. On the other hand, a difference of electrical potential modifies the aggregation of molecules. The experiments of Lippman are well known to you. He has constructed an electrometer, and even an electrical machine, which depend upon the principle that the superficial energy of a surface of mercury covered with acidulated water is modified when a difference of electrical potential is produced at the limiting surfaces. I have lately noticed a striking illustration of the modification of superficial energy by a difference of electrical potential. The experiment can be performed in this way: Fill the lower part of a glass jar with clean mercury, pour a saturated solution of common salt upon the mercury, hang in the salt solution a carbon plate, and connect this plate with a battery of four or five Bunsen cells; and, on connecting an iron wire with the other pole of the battery, touch the surface of the mercury. An amalgam will be speedily formed and chlorine gas given off. After a slight film of this amalgam has been formed on the mercury, remove the iron wire, and then immerse it slowly in salt-and-water. Even at a distance of six inches from the mercury, and far below the carbon electrode, the surface of mercury will be disturbed by the difference of electrical potential, and a commotion, which might be called an electrical storm, will be observed upon its surface. Now, these manifestations of what is called superficial energy—that is, the energy manifested at the surface of separation of any two media, and the effect of electricity upon this superficial energy—afford, it seems to me, much food for thought. There have always been two parties in electricity—one which maintains that electricity is due to the contact of dissimilar substances, and the other party which believes that the source of electrical action must be sought in chemical action. Thus, according to one party, the action of an ordinary voltaic cell is due to the contact, for instance, of zinc with copper, the acid or solution of the cell merely acting as the connecting link between the two. According to the other party, it is to the difference of chemical action on the two metals of the connecting liquid that we must attribute the rise and continuance of the electrical current. It has always seemed to me that these two parties are like the knights in the story, who stood facing opposite sides of a shield, each seeing but one side, one protesting that the shield was silver and the other that it was gold, whereas it was both silver and gold.
The electro-motive force of a voltaic cell is undoubtedly due to the intrinsic superficial manifestation of energy. When two dissimilar metals are placed in connection with each other, either directly or through the medium of a conducting liquid, the chemical action of the liquid brings new surfaces of the metals constantly in contact. Moreover, we have the difference of superficial energy between the liquid and the two metals. So that our expression for electro-motive force is far from being a simple one; it contains the sum of the several modifications of superficial energy at the surfaces of the two metals, and at the two boundaries of the liquid and the metals.
Let us turn now to the subject of thermo-electricity. Here we have again a development of electro-motive force by the mere contact of two metals, when the junctions of the metals are at different temperatures. There is no connecting liquid here, but the surface of one metal rests directly upon that of the other. The electrical current that arises is due to the difference of superficial energy manifested at the surfaces of the two junctions. We know that the action is on the surface, for the dimensions of the junctions do not affect the electro-motive force. Suppose that we should make the metals so thin that an ultimate molecule of iron should rest against an ultimate molecule of copper. Should we not arrive at a limit, at a definite temperature of the conversion of molecular vibration into electrical energy, and, also, when our theory is perfected, of the number of molecules along a linear line of copper against a linear line of zinc which can produce a current of electricity of a given strength? I have often thought that the jostling, so to speak, of these ultimate molecules of two metals at definite temperatures might form a scientific unit of electro-motive force in the future science of physical chemistry. Look at the great field for investigation there is in the measurement of what we call electro-motive force, both in voltaic electricity and in thermo-electricity. The astronomer measures the positions of the stars and their light, and tabulates the enormous volumes of results from year to year, in order to ascertain some great law or laws of the possible changes of the entire stellar universe—some sweeping onward through space. Is it not fully as important that, in our physical laboratories, we should organize our routine work, and provide some great generalizer, like Sir Isaac Newton, with sufficient data of electro-motive force, or, as I prefer to call it, the relations of superficial energy, in order that the relations between this energy and the ultimate motions of the molecular worlds may become better known to us?
When the world was evolved from the first nebulous stage, a portion of the atoms remained more or less free in the gaseous state, another portion became more or less limited in organic forms, and another portion were tightly compressed into solids more or less elastic. This elasticity is thought by some to be an evidence of very rapid motion through all these various aggregations of matter—or shall we say different manifestations of motion? for some also believe that our ideas of matter result merely from a perception of motion. Shall we affirm that there is some relation between elasticity and electricity? I do not think that we are prepared to do so, for some elastic bodies are good conductors and some are poor conductors of electricity. We can see dimly, however, that there is a great field in molecular physics, in which elasticity and superficial energy and difference of electrical potential shall be treated together.
I have tried various experiments upon the electro-motive force of alloys. By means of an alloy we can apparently modify the superficial energy at the surface of a solid. Thus, an alloy with a parent metal will give a varying electro-motive force. If we could be sure that an alloy is always a definite chemical composition, and not a more or less mechanical admixture, it seems as if we could get closer to the seat of electro-motive force by a number of quantitative measurements. Unfortunately, the physical nature of alloys is not definitely known, and there is little coherence or regularity in our measurements of their electro-motive force. Still, there is a great field in the study of the electro-motive force of alloys. We can modify the superficial energy of metals, not only by melting metals together, but also by grinding them to a very fine powder, and compressing them again by powerful means into solids more or less elastic, and then examining their superficial energy, which is manifested as electro-motive force. I am still engaged upon researches of this nature, and, if the work is not brilliant, I hope that it will result in the accumulation of data for future generalization.
"We can not treat the manifestation of electro-motive force in the voltaic cell apart from the manifestation of electro-motive force in the differently heated junctions of a thermo-electric junction. In both cases there is a difference of manifestation of superficial energy; and in thermo-electricity we can also modify this energy by making alloys. The subject of thermo-electricity has been eclipsed by the magnificent development of the dynamo-electric machine, but we may return to thermo-electricity as a practical source of electricity. I have been lately occupied in endeavoring to modify the difference of potential of two thermo-electric junctions, by raising one junction to a very high temperature under great pressure; for it is well known that the melting-point of metals is raised by great pressure. If the metal still remains in the solid state under great temperature and great pressure, can we not greatly increase the electro-motive force which results from the difference of superficial energy manifested at the two junctions?
When an electrical current is passed through two thermo-electric junctions, it cools the hot one and heats the cool one. Moreover, you are well acquainted with Thomson's discovery that a current of electricity, in passing through a metal, carries heat, so to speak, with it—in one direction in iron and in another in copper. Do we not see here a connection between the manifestation of superficial energy in liquids and the effect of a difference of potential upon it, and the manifestations of thermo-electro-motive force and the effect of differences of electrical potential? It is curious and suggestive that, in applying the reasoning of Carnot's cycle to the effect of a difference of electrical potential on the superficial energy at the surface of separation of two liquids, one is led to the equations which express in thermo-dynamics the Peltier and Thomson effects.
In thus looking for the seat and origin of electrical action, how much have we discovered? It is evident that our knowledge of electricity will increase with our knowledge of molecular action, and our knowledge of molecular action with that which we call attractive force. It is somewhat strange that, although we are so curious in regard to electricity, we seldom reflect that gravitation is as great a mystery as electrical attraction. What is this force which acts instantly through space, and which holds our entire solar system together? "We know only its simple law—that it attracts bodies directly as their masses and inversely as the square of their distance; but we do not know what relation it bears to electrical force or magnetic force. Here is a field in which we are to push back our boundary of electrical knowledge. I will not call it electrical knowledge, but rather our knowledge of the great doctrine of the conservation of energy. What is the relation between electricity, magnetism, and oravitation, and what we call the chemical force of attraction? It seems to me that this is the question which we must strive to answer; but, when this question is answered, shall we not be as far as ever from the answer to the question, "What is electricity?"
The question of the connection between electricity and gravitation dwelt much in Faraday's thoughts. It is interesting to recall the experiments which he instituted to discover if there is any connection between these great manifestations of force. In his first experiment he suspended vertically an electro-magnet, which was connected with a delicate galvanometer, and let various non-magnetic bodies, such as brass bodies, pieces of stone and crystals, fall through the center of this electro-magnet, thinking that there might possibly be a reaction from the influence of gravitating force on the falling body which would be manifested as an electrical current. He did not, however, obtain the slightest electrical disturbance which might not have been caused by simple electrical induction. He then arranged a somewhat complicated piece of apparatus by means of which a body could be moved alternately with the direction of gravitation and against it, and the terminals of a galvanometer were so connected that the intermittent effects, if they existed, could be integrated or summed up. He failed, however, to find the slightest relation between gravitation and electricity, and he closes his account of his experiments with these words: "Here end my trials for the present. The results are negative. They do not shake my strong feeling of the existence of a relation between gravity and electricity, though they give no proof that such a relation exists."
Since Faraday's time no connection or relation has been found except in the similarity of the law of inverse squares. I have often reflected upon these experiments of Faraday, and have asked myself, Was the direction in which he experimented the true direction to look for a possible relation; and can not the refined instruments and methods of the electrical science of the present aid us in more promising lines of research? Should we not expect that, when two balls of copper, for instance, are suddenly removed from each other, a difference of electrical potential should manifest itself, and that the electrical force thus developed should be opposed to our endeavor to overcome the attractive force between the masses of copper? Moreover, when we force the copper balls together, should we not expect that an electrical charge should be developed of such a nature as to oppose our motion? And thus in these mutual relations, which are apparently consistent with the doctrine of the conservation of energy, should we not expect to find the relation which we are in search of? Our experiment, therefore, would have to be conducted in this way: We should carefully insulate our two copper masses, estimate the effects that would be due in any way to cutting the magnetic lines of force of the earth, and then with a delicate electrometer, the masses having been placed in a vacuum to get rid of the effect of friction of the air, we should proceed to test their electrical relations. This experiment also gives negative results, but may we not try it under better conditions than I have been able to devise? If we could prove that whenever we disturb the relative position of bodies, or break up the state of aggregation of particles, we create a difference of electrical potential, and, moreover, if we could discover that the work that this electrical potential can perform, together with the heat that is developed by the process, is the complete work that is done on the system against attractive force, whether expressed in gravitation attractive force, or as so-called chemical attractive force, we should greatly extend our vision of the relation of natural phenomena. In thus pursuing the line of argument of my address, I venture to state an hypothetical law which it seems to me is at least plausible in the present state of electrical science, and may serve as a scaffolding to be taken down when experiment shall have properly proportioned the edifice.
This hypothetical law I should state as follows: "Whenever the force of attraction between masses or molecules is modified in any way, a difference of electrical potential results."
In what I may therefore call the physical chemistry of the future, may we not expect that in the reactions we must express the equivalent of the difference of electrical potential in the summation of the entire work which is done? I can make my meaning clearer by referring to an experiment of Him by which he obtained a fair value for the mechanical equivalent of heat. In principle it is this: A heavy weight falls upon a lead vessel which contains a given amount of water at a definite temperature. The lead vessel suffers compression by the blow, and the water is raised in temperature. It is found, on properly estimating the amount of heat taken up by the lead and the loss radiated during the experiment, that the heat produced by the blow is the equivalent of its mechanical work. Suppose now that the vessel containing the water should be made of two metals of about the same specific heat or capacity for absorbing heat, and suppose that wires should connect the two different metallic portions with a similar vessel containing water. We should have here two thermo-electric junctions at the same temperature. When the weight falls upon one junction and heats it with its contained water, we have not only the heated water but also an electrical current; it is evident, therefore, that we should be able to heat the water more when the wire between the two vessels is cut, than when there is a metallic circuit between them, for a part of the energy of the falling weight has become converted into an electrical current. At the terminals of the cut wires there is a difference of electrical potential created for an instant, which, however, instantly disappears. What is the equivalent of the disappearance of this difference of potential? Is it not in the closed circuits through the masses of the metals, a part of which, it is true, becomes sensible heat, but another portion may become latent heat or do internal work among the molecules?
Moreover, is it not reasonable to suppose that certain anomalies which we now find in the determinations of specific heats of complicated aggregation of molecules are due to our failure to estimate the electrical equivalent of the movements and interchanges of the molecules? Let us take, again, the case of friction between two pieces of wood: is it not possible that the friction is the electrical attraction which results from the endeavor to move the adjoining particles of wood in the two pieces asunder? Let us remember, in our endeavor to connect the phenomenon of superficial energy with electrical manifestations, that the friction between two surfaces is modified by keeping these surfaces at a difference of electrical potential. In Edison's motophone, by means of which the voice of one speaking in New York could be made audible to this audience, we see this exemplified in a very striking manner. A platinum point connected with one pole of a battery rubs upon a revolving cylinder of chalk, which is simply moistened with water and is connected with the opposite pole of the battery. The friction between the two is modified in unison with the changes in electrical potential of the battery; and a diaphragm in connection with the platinum point responds to these changes in the friction, and therefore to a transmitter placed anywhere in the electrical circuit.
My own studies have been chiefly in the direction of thermo-electricity, and in the subject of the electrical aspect of what we call superficial energy. I think there is a great field here—in which a large crop of negative results can be reaped—but these negative results I can not regard entirely as thistles. I have tried the following experiment, on the hypothesis that an electrical difference of potential in changing the relations of molecules might modify the heat that is radiated from a surface. I have endeavored to discover whether an electrical current first cools a conductor before it heats it, as we might expect if the molecules being restrained in any way could not radiate as much energy into space as they could under the same difference of temperature, when not submitted to the action of an electrical difference of potential. I have reaped only a thistle so far from this investigation, but I shall continue it. I have deposited copper in the magnetic field and outside the magnetic field, and have endeavored to ascertain the thermo-electric relations of these layers of copper, and have apparently discovered—I say apparently, for such experiments require a large number of trials, and I have made thus far only a limited number—that there is a difference of superficial energy between the surface in which the molecules of copper have been subjected to a strong attractive force while they were being deposited, and those molecules which have been only under the influence of ordinary gravitation force.
The experiments which I have tried have continually deepened in me the belief that any change in the state of aggregation of particles—in other words, any change which results in a modification of attracting force—whether gravitative or the commonly called chemical attracting forces, results in an electrical potential; and, conversely, that the passage of electricity through any medium produces a change of aggregation of the molecules and atoms. Professor Schuster, in a late number of "Nature" (July 3, 1884, page 230), gives some of the results of his recent investigation of gases subjected to electrical discharges, and believes himself justified in making the following hypothesis: "In a gas the passage of electricity from one molecule to another is always accompanied by an interchange of the atoms composing the molecule; the molecules are always broken up at the negative pole," and in his comments upon this law he remarks that a molecule of mercury consists of a single atom; but mercury has a very brilliant spectrum: this would seem to militate against the hypothesis. On the other hand, if an essential part of the glow discharge is due to the breaking up of the molecules, we might expect mercury-vapor to present other and much simpler phenomena than other vapors. This is the case, for if mercury-vapor is sufficiently free from air, the electrical discharge through it shows no negative glow, no dark spaces, and no stratifications. In reflecting upon experiments of this nature, can not we believe that, if we could systematically break up the arrangement of the atoms in the molecules of any substance, we could produce a difference of electrical potential? Our instrumental means are probably too coarse to enable us to follow the track of such splitting of the molecules. We are like blind men in a great field of energy striving to ascertain the configuration about us with only three senses—the galvanometer sense, the electrometer sense, and the voltameter sense. Suppose you add to the equipment of such blind men a magnetic sense, or an attractive-force sense. Suppose such a blind man could perceive the equivalence of our thoughts in electrical and magnetic relations, as we now sec a manifestation of equivalence of mechanical work when a lighthouse lamp bursts upon our sight. Suppose such a person could become sensible of every change among atoms and molecules. Suppose that the quick passing of what we call life from the body into another shape or state of existence should be sensible as a reaction in electrical and magnetic effects. Such a person could then see the quick ebb and flow and interchanges of attractive forces as we now see the play of colors. Have you ever reflected that we may possibly have some day an electrical spectrum—perhaps I should call it an attracting-force spectrum—in which the electro-magnetic manifestations of energy shall be spread out and differentiated, just as that part of the energy which we receive from the sun and which we call light and heat is now dispersed in the visible solar spectrum? We regard to-day the manifestations of the different colors of bodies—the tints of the objects in the room—as the visible expression of the great law of conservation of energy. The energy which we have received from the sun is making interchanges and is modified by the different molecular structure of the different objects. Thus, a red body has absorbed, so to speak, certain wave-lengths of energy, and has transmitted or reflected back only the red or long waves of energy. The rest of the energy has been devoted to molecular work which does not appeal to us as light or even in certain cases as heat. If we suppose that radiant energy is electro-magnetic, can not we suppose that it is absorbed more readily by some substances than by others, that its energy is transformed so that with the proper sense we could perceive what might be called electrical color; or, in other words, have an evidence of other transformations of radiant energy other than that which appeals to us as light and color?
I have thus far conducted you over a field that, in comparison with what lies before us, seems indeed barren and churlish of results. Have we, then, nothing upon which we can congratulate ourselves? I can only reply by pointing to the rich practical results which you can see in the fine electrical exposition which we owe to the energy and liberality of the citizens of Philadelphia. Although we must glory in this exposition, it is the duty of the idealist to point out the way to greater progress and to greater intellectual grasp.
Perhaps we have arrived at that stage in our study of electricity where our instruments are too coarse to enable us to extend our investigations. Yet how delicate and efficient they are! Compare the instruments employed by Franklin, and even by Faraday, with those which are in constant use to-day in our physical laboratories. Franklin, by the utmost effort of his imagination, could not conceive, probably, of a mirror-galvanometer that can detect the electrical action of a drop of distilled water on two so-called chemically pure platinum plates, or of a machine that can develop from the feeble magnetism of the earth a current sufficiently strong to light the city of Philadelphia. Let him who wanders among the historical physical instruments of many of our college collections stand before the immense frictional electrical machine of Franklin's day, or gaze upon the rude electrometers and galvanometers of that time, and contrast Franklin's machine with the small Toepler-Holtz electrical machine which with a tenth of the size gives a spark ten times as strong as Franklin's; or the electrometers and galvanometers of Faraday with the mirror-galvanometers and electrometers of Sir William Thomson. Yet, at the same time, let such an observer think of the possibilities of the next fifty years, for the advance of science is not in a simple proportion to the time, and the next fifty years will probably see a far greater advance than the one hundred years since the date of Franklin's electrical work has seen. Is not the state of our imagination like that of the shepherd-boy who lies upon his back, looking up at the stars of heaven, and trying to imagine what is beyond the stars? The only conclusion is that there is something far more than we have ever beheld. Is not the physicist of the future to have instruments delicate enough to measure the heat equivalent of the red and the yellow, the blue and the violet rays of energy?—instruments delicate enough to discover beats of light as we now discover those of sound—an apparatus which will measure the difference of electrical potential produced by the breaking up of composite grouping of molecules? The photographer of to-day speaks, in common language, of handicapping molecules by mixing gums with his bromide of silver, in order that their rate of vibration may be affected by the long waves of energy. Shall we not have the means of obtaining the mechanical equivalent of such handicapped vibrations? Or, turning to practical science, let us reflect upon the modern transmitter and the telephone, and contrast these instruments with the rude, so-called lover's telephone, which consists of two disks connected by a string or wire. What an almost immeasurable advance we see here! Would it not have been as difficult for Franklin to conceive of the electrical transmission of speech as for the shepherd-boy to conceive of other stars as far beyond the visible stars as the visible stars are from the earth?
Yes, we have advanced; but you will perceive that I have not answered the question, which filled the mind of Franklin, and which fills men's minds to day, "What is electricity?" If I have succeeded in being suggestive, and in starting trains of thought in your minds which may enlighten us all upon this great question, I have indeed been fortunate.
- Address before Section B, of the American Association for the Advancement of Science, Philadelphia, September 4, 1884.