TOC | Previous Section:Appendix A | Next Section:Appendix CA History of Wireless Telegraphy (2nd edition, revised), J. J. Fahie, 1901, pages 277-291:
PROF. HENRY ON HIGH TENSION ELECTRICITY BEING CONFINED TO THE SURFACE OF CONDUCTING BODIES, WITH SPECIAL REFERENCE TO THE PROPER CONSTRUCTION OF LIGHTNING-RODS.
(Extracted from the 'Journal of the Telegraph,' New York, Sept. 1, 1877.)
WASHINGTON, March 11, 1876.
DEAR SIR,--In answer to your letter of the 7th inst., I have to say that the discrepancy which exists as to the question whether electricity passes at the surface or through the whole capacity of the rod has arisen principally from experiments on galvanic electricity, which, having little or no repulsive energy, passes through the whole substance of the rod, and also from experiments in which a very large quantity of frictional electricity is transmitted through a small wire in this case the metal is resolved into its elements and reduced to an impalpable powder.
In the case, however, of the transmission of atmospheric electricity through a rod of sufficient size to transmit the discharge freely, there can be no doubt that it tends to pass at the surface, the thickness of the stratum of electricity varying with the diameter of the rod and the amount and the intensity of the charge.
To test this by actual experiment I made the following arrangement: through a gun-barrel about 2 feet in length a copper wire was passed, the ends projecting. The middle of the wire in the barrel was coiled into the form of a magnetising spiral, and the ends of the gun-barrel were closed with plugs of tinfoil, so as to make a perfect metallic connection between the wire and the barrel. On the outside of the barrel another magnetising spiral was placed, the whole arrangement being shown in the sketch.
A powerful charge was now sent through the copper wire from a Leyden jar of about two gallons' capacity. The needle within the barrel showed not the least sign of magnetism, while the one on the outside was strongly magnetic.
From this experiment I conclude that a gas-pipe can convey an ordinary charge of electricity from the clouds as well as a solid rod of the same diameter.
The repulsive energy of the electrical discharge at right angles to the axis remains of the same intensity as in the case of a statical charge. This I have shown to be the case by drawing sparks of considerable intensity from a conductor, one end of which was connected with the ground while sparks were thrown on the other end from a large prime conductor. This spark is of a peculiar character, for though it gives a pungent shock and sets fire to combustible substances, such as an electrical pistol, it does not affect a sensitive gold-leaf electrometer. The fact is, it consists of two sparks, the one negative and the other positive. The rod during the transmission of the electricity through it is charged + at the upper end, and immediately in advance of this point it is charged - by induction, and the electricity passes through it in the discharge in the form of a series of + and - waves.--Yours very truly, JOSEPH HENRY, Sec. Smithsonian Inst. Prof. R. C. KEDZIE, Lansing, Michigan.
WASHINGTON, April 15, 1876.
DEAR SIR,--Your letter was received by due course of mail, but a press of business connected with the preparation of the Annual Report for 1875 and the Lighthouse Board has prevented an earlier reply.
I have now to say that, as far as I know, I am the only person who has made a special study of the conduction of frictional electricity in regard to lightning-rods. It has long been established by Coulomb and others that the electricity of a charged conductor exists in a thin stratum at the surface, and this is a necessary consequence of the repulsion of electricity for itself, every particle being repelled from every other as far as possible. From this it was hastily assumed that electricity in motion also moves at the surface; but this was an inference without physical proof until I commenced the investigation. I found from a series of experiments that frictional electricity--that is, electricity of repulsive energy, such as that from the clouds--does pass at the surface, but that galvanic electricity, the kind to which Faraday, Daniell, De La Rive, and others refer, passes through the whole capacity of the conductor. This latter fact, however, was previously established by others. I further found that whenever a charge of electricity was thrown on a rod explosively, however well connected the rod was with the earth, it gave off sparks in the course of its length sufficient to fire an electric pistol and light flocculent substances. I also found that, in sending a powerful discharge from a battery of nine jars through a wide plate, no electricity passed along the middle of the plate, but that it was accumulated in its passage at the edges.
From all my study of this subject I do not hesitate to say that the plan I have given of lightning-rods is the true one, and that a tube of a sufficient degree of thickness serves to conduct the electricity as well as a solid mass, provided the thickness is sufficient to give free conduction. A very heavy charge sent through a wire frequently deflagrates it, but no discharge from the clouds, of which I have any knowledge, has ever sufficed to deflagrate a gas-pipe of an inch in diameter.
The plan of increasing the surface of a rod by converting the metal into a ribbon is objectionable. It tends to increase the power of the lateral discharge, and gives no increase of conducting power.
Another fallacy is much insisted on--viz., the better conduction of copper than iron. It is true that copper is a better conductor of galvanic electricity, which pervades the whole mass, but in regard to frictional electricity the difference in conducting capacity is too small to be of any importance. Iron is sufficiently good in regard to conduction, and withstands deflagration better than copper: besides this, it is much cheaper.--Yours truly, JOSEPH HENRY.
Prof. R. C. KEDZIE.
ON MODERN VIEWS WITH RESPECT TO THE NATURE OF ELECTRIC CURRENTS.
Substance of a lecture by Prof. H. A. Rowland, American Institute of Electrical Engineers, May 22, 1889. 1
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How great, then, the difference between a current of water and a current of electricity! The action of the former is confined to the interior of the tube, while that of the latter extends to great distances on all sides, the whole of the space being agitated by the formation of an electric current in any part. To show this agitation, I have here two large frames with coils of wire around them. They hang face to face about 6 feet apart. Through one I discharge this Leyden jar, and immediately you see a spark at a break in the wire of the other coil, and yet there is no apparent connection between the two. I can carry the coils 50 feet or more apart, and yet, by suitable means, I can observe the disturbances due to the current in the first coil.
The question is forced upon us as to how this action takes place. How is it possible to transmit so much power to such a distance across apparently unoccupied space? According to our modern theories of physics, there must be some medium engaged in this transmission. We know that it is not the air, because the same effects take place in a vacuum, and therefore we must fall back on that medium which transmits light, and which we have named the ether--that medium which is supposed to extend unaltered throughout the whole of space, whose existence is very certain, but whose properties we have yet but vaguely conceived.
I cannot in the course of one short hour give even an idea of the process by which the minds of physicists have been led to this conclusion, or the means by which we have finally completely identified the ether which transmits light with the medium which transmits electrical and magnetic disturbances. The great genius who first identified the two is Maxwell, whose electro-magnetic theory of light is the centre around which much scientific thought is to-day revolving, and which we regard as one of the greatest steps by which we advance nearer to the understanding of matter and its laws. It is this great discovery of Maxwell which allows me to attempt to explain to you the wonderful events which happen everywhere in space when one establishes an electric current in any other portion.
In the first place, we discover that the disturbance does not take place in all portions of space at once, but proceeds outward from the centre of the disturbance with a velocity exactly equal to the velocity of light; so that when I touch these wires together so as to complete the circuit of yonder battery, I start a wave of ethereal disturbance which passes outward with a velocity of 185,000 miles per second, and continues to pass outwards for ever, or until it reaches the bounds of the universe. And yet none of our senses informs us of what has taken place unless sharpened by the use of suitable instruments. Thus, in the case of these two coils of wire, suspended near each other, when the wave from the primary disturbance reaches the second coil we perceive the disturbance by means of the spark formed at the break in the coil. Should I move the coils farther apart, the spark in the second coil would be somewhat delayed, but the distance of 185,000 miles would be necessary before this delay could amount to as much as one second. Hence the effects we observe on the earth take place so nearly instantaneously that the interval of time is very difficult to measure, amounting in the present case to only 1/150000000th of a second.
It is impossible for me to prove the existence of this interval, so infinitesimal is it, but I can at least show you that waves have something to do with the action observed. For instance, I have here two tuning-forks mounted on sounding-boxes and tuned to exact unison. I sound one and then stop its vibrations with my hand; instantly you hear that the other is in vibration, caused by the waves of sound in the air between the two. When, however, I destroy the unison by fixing this piece of wax on one of the forks, the action ceases.
Now, this combination of a coil of wire and a Leyden jar forms a vibrating system of electricity, and its time of vibration is about 10,000,000 times a second. Here is another combination of coil and jar, the same as the first, and therefore its time of vibration is the same. You see how well the experiment works, because the two are in unison. But let me take away this second Leyden jar, thus destroying the unison, and you see that the sparks instantly cease. Replacing it, the sparks reappear. Adding another on one side, they disappear again, only to reappear when the system is made symmetrical by placing two on each side.
This experiment and that of the tuning-forks have an exact analogy to one another. In each we have two vibrating systems connected by a medium capable of transmitting vibrations, and they both come under the head of what we know as sympathetic vibrations. In the one case, we have two mechanical tuning-forks connected by the air; in the other, two pieces of apparatus, which we might call electrical tuning-forks, connected by the ether. The vibrations in one case can be seen by the eye or heard by the ear, but in the other case they can only be perceived when we destroy them by making them produce a spark. The fact that we are able to increase the effect by proper tuning demonstrates that vibrations are concerned in the phenomenon. This can, however, be separately demonstrated by examining the spark by means of a revolving mirror, when we find that it is made up of many successive sparks corresponding to the successive backward and forward movements of the current.
Thus, in the case of a charged Leyden jar whose inner and outer coatings have been suddenly joined by a wire, the electricity flows back and forth along the wire until all the energy originally stored up in the jar has expended itself in heating the wire or the air where the spark takes place, and in generating waves of disturbance in the ether which move outward into space with the velocity of light. These ethereal waves we have demonstrated by letting them fall on this coil of wire, causing the electrical disturbance to manifest itself by electric sparks.
I have here another more powerful arrangement for producing electro-magnetic waves of very long wave length, each one being about 500 miles long. It consists of a coil within which is a bundle of iron wires. On passing a powerful alternating current through the coil the iron wires are rapidly magnetised and demagnetised, and send forth into space a system of electro-magnetic waves at the rate of 360 in a second.
Here also I have another piece of apparatus for sending out the same kind of electro-magnetic waves, and on applying a match we start it also into action. But the last apparatus is tuned to so high a pitch that the waves are only 1/50000 inch long, and 55,000,000,000,000 are given out in one second. These short waves are known by the name of light and radiant heat, though the name radiation is more exact. Placing any body near the lamp so that the radiation can fall on it, we observe that when the body absorbs the rays it is heated by them. Is it not possible for us to get some substance to absorb the long (or electro-magnetic) waves of disturbance, and so obtain a heating effect? I have here such a substance in the shape of a sheet of copper, which I fasten on the face of a thermopile, and I hold it where these waves are strongest. As I have anticipated great heat is generated by their absorption, and soon the plate of copper becomes very warm, as we see by this thermometer, by feeling it with the hand, or even by the steam from water thrown upon it. In this experiment the copper had not touched the coil or the iron wire core, although if it did they are very much cooler than itself. The heat has been produced by the absorption of the waves in the same way as a blackened body absorbs the rays of shorter wave length from the lamp.
In these experiments, so far, the wave-like nature of the disturbance has not been proved. We have caused electric sparks, and have heated the copper plate across an interval of space, but have not in either of these cases proved experimentally the progressive nature of the disturbance.
A ready means of experimenting on the waves, obtaining their wave length and showing their interferences, has hitherto been wanting. This deficiency has been recently supplied by Prof. Hertz, of Carlsruhe.
I scarcely know how to present this subject to a non-technical audience and make it clear how a coil of wire with a break in it can be used to measure the velocity and length of ethereal waves. However, I can but try. If the waves moved very slowly, we could readily measure the time the first coil took to affect the second, and show that this time was longer as the distance was greater. But it is absolutely inappreciable by any of our instruments, and another method must be found. To obtain the wave length Prof. Hertz used several methods, but that by the formation of stationary waves is the most easily grasped. I hold in my hand one end of a spiral spring, which makes a heavy and flexible rope. As I send a wave down it, you see that it is reflected at the farther end, and returns again to my hand. If, how ever, I send a succession of waves down the rope, the reflected waves interfere with the direct ones, and divide the rope into a succession of nodes and loops which you now observe. So, a series of sound waves, striking on a wall, forms a system of stationary waves in front of the wall. Indeed we can use any waves for this purpose, even ethereal waves. With this in view Prof. Hertz established his apparatus in front of a reflecting wall, and observed the nodes and loops by the sparks produced in a ring of wire, somewhat resembling the coil I have been using, but much smaller. It is impossible for me to repeat this experiment before you, as it is a very delicate one, and the sparks produced are almost microscopic. Indeed I should have to erect an entirely different apparatus, as the waves from the one before me are nearly a quarter-mile long. To produce shorter waves we must use apparatus very much smaller--tuned, as it were, to a higher pitch, so that several stationary waves, or nodes and loops, of a few yards long could be obtained in the space of this room.
The testing coil would then be moved to different parts of the room, and the nodes would be indicated by the disappearance of the sparks, and the loops by the greater brightness of them. The presence of stationary waves would thus be proved, and their half-wave length found from the distance from node to node, for stationary waves can always be considered as produced by the interference of two waves advancing in opposite directions.
The closing of a battery circuit, then, and the establishment of a current of electricity in a wire, is a very different process from the formation of a current of water in a pipe, though after the first shock the laws of the flow of the two are very much alike. Furthermore, the medium around the current of electricity has very strange properties, showing that it is accompanied by a disturbance throughout space. The wire is but the core of the disturbance, which latter extends indefinitely in all directions.
One of the strangest things about it is that we can calculate with perfect exactness the velocity of the wave propagation and the amount of the disturbance at every point and at any instant of time; but as yet we cannot conceive of the details of the mechanism which is concerned in the propagation of an electric current. In this respect our subject resembles all other branches of physics in the partial knowledge we have of it. We know that light is the undulation of the luminiferous ether, and yet the constitution of the latter is unknown. We know that the atoms of matter can vibrate with purer tones than the most perfect piano, and yet we cannot even conceive of their constitution. We know that the sun attracts the planets with a force whose law is known, and yet we fail to picture to ourselves the process by which it takes our earth within its grasp at the distance of many millions of miles and prevents it from departing for ever from its life-giving rays. Science is full of this half-knowledge.
So far we have considered the case of alternating electric currents in a wire connecting the inner and outer coatings of a Leyden jar. The invention of the telephone, by which sound is carried from one point to another by means of electrical waves, has forced into prominence the subject of these waves. Furthermore, the use of alternating currents for electric lighting brings into play the same phenomenon. Here, again, the difference between a current of water and a current of electricity is very marked. A sound wave, traversing the water in the tube, produces a to-and-fro current of water at any given point. So, in the electrical vibration along a wire, the electricity moves to and fro along it in a manner somewhat similar to the water, but with this difference the disturbance from the water-motion is confined to the tube, and the oscillation of the water is greatest in the centre of the tube; while in the case of the electric current the ether around the wire is disturbed, and the oscillation of the current is greatest at the surface of the wire and least in its centre. The oscillations in the water take place in the tube without reference to the matter outside the tube, whereas the electric oscillations in the wire are entirely dependent on the surrounding space, and the velocity of the propagation is nearly independent of the nature of the wire, provided it is a good conductor.
We have then in the case of electrical waves along a wire a disturbance outside the wire and a current within it, and the equations of Maxwell allow us to calculate these with perfect accuracy and give all the laws with respect to them.
We thus find that the velocity of propagation of the waves along a wire, hung far away from other bodies and made of good conducting material, is that of light, or 185,000 miles per second; but when it is hung near any conducting matter, like the earth, or enclosed in a cable and sunk into the sea, the velocity becomes much less. When hung in space, away from other bodies, it forms, as it were, the core of a system of waves in the ether, the amplitude of the disturbance becoming less and less as we move away from the wire. But the most curious fact is that the electric current penetrates only a short distance into the wire, being mostly confined to the surface, especially where the number of oscillations per second is very great.
The electrical waves at the surface of a conductor are thus, in some respects, very similar to the waves on the surface of water. The greatest motion in the latter case is at the surface, while it diminishes as we pass downwards and soon becomes inappreciable. Furthermore, the depth to which the disturbance penetrates into the water increases with increase of. the length of the wave, being confined to very near the surface for very short waves. So the disturbance in the copper penetrates deeper as the waves and the time of oscillation are longer, and the disturbance is more nearly confined to the surface as the waves become shorter. 2 There are very many practical applications of these theoretical results for electric currents. The most obvious one is to the case of conductors for the alternating currents used in producing the electric light. We find that when these are larger than about half an inch diameter they should be replaced by a number of conductors less than half an inch diameter, or by strips about a quarter of an inch thick, and of any convenient width.
Prof. Oliver Lodge has recently drawn attention to another application of these results--that is, to lightning-rods. Almost since the time of Franklin there have been those who advocated the making of lightning-rods hollow in order to increase the surface for a given amount of copper. We now know that these persons had no reason for their belief, as they simply drew the inference that electricity at rest is on the surface. Neither were the advocates of the solid rods quite correct, for they reasoned that electricity in a state of steady flow occupies the whole area of the conductor equally. The true theory, we now know, indicates that neither party was entirely correct, and that the surface is a very important factor in the case of a current of electricity so sudden as that from a lightning discharge. But increase of surface can best be obtained by multiplying the number of conductors, rather than making them flat or hollow. Theory indicates that the current penetrates only one-tenth the distance into iron that it does into copper. As the iron has seven times the resistance of copper, we should need seventy times the surface of iron that we should of copper. Hence I prefer copper wire about a quarter of an inch diameter and nailed directly to the house without insulators, and passing down the four corners, around the eaves, and over the roof, for giving protection from lightning in all cases where a metal roof and metal down-spouts do not accomplish the same purpose.
Whether the discharge of lightning is oscillatory or not does not enter into the question, provided it is only sufficiently sudden. I have recently solved the mathematical problem of the electric oscillations along a perfectly conducting wire joining two infinite and perfectly conducting planes parallel to each other, and find that there is no definite time of oscillation, but that the system is capable of vibrating in any time in which it is originally started. The case of lightning between a cloud of limited extent and the earth along a path through the air of great resistance is a very different problem. Both the cloud and the path of the electricity are poor conductors, which tends to lengthen the time. If I were called on to estimate as nearly as possible what took place in a flash of lightning, I would say that I did not believe that the discharge was always oscillating, but more often consisted of one or more streams of electricity at intervals of a small fraction of a second, each one continuing for not less than 1/100000 second. An oscillating current with 100,000 reversals per second would penetrate about 1/60 inch into copper and 1/600 inch into iron. The depth for copper would constitute a considerable proportion of a wire ¼ inch diameter, and as there are other considerations to be taken into account, I believe it is scarcely worth while making tubes, or flat strips, for such small sizes.
It is almost impossible to draw proper conclusions from experiments on this subject in the laboratory, such as those of Prof. Oliver Lodge. 3 The time of oscillation of the current in most pieces of laboratory apparatus is so very small, being often the 1/100000000 of a second, that entirely wrong inferences may be drawn from them. As the size of the apparatus increases, the time of oscillation increases in the same proportion, and changes the whole aspect of the case. I have given of a second as the shortest time a lightning-flash could probably occupy. I strongly suspect it is often much greater, and thus departs even further from the laboratory experiments of Prof. Lodge, who has, however, done very much towards drawing attention to this matter and showing the importance of surface in this case. All shapes of the rod with equal surface are not however, equally efficient. Thus, the inside surface of a tube does not count at all. Neither do the corrugations on a rod count for the full value of the surface they expose, for the current is not distributed uniformly over the surface; but I have recently proved that rapidly alternating currents are distributed over the surface of very good conductors in the same manner as electricity at rest would be distributed over them, so that the exterior angles and corners possess much more than their share of the current, and corrugations on the wire concentrate the current on the outer angles and diminish it in the hollows. Even a flat strip has more current on the edges than in the centre.
For these reasons, shape, as well as extent of surface, must be taken into account, and strips have not always an advantage over wires for quick discharges.
The fact that the lightning-rod is not melted on being struck by lightning is not now considered as any proof that it has done its work properly. It must, as it were, seize upon the discharge, and offer it an easier passage to the earth than any other. Such sudden currents of electricity we have seen to obey very different laws from continuous ones, and their tendency to stick to a conductor and not fly off to other objects depends not only on having them of small resistance, but also on having what we call the self-induction as small as possible. This latter can be diminished by having the lightning-rod spread sideways as much as possible, either by rolling it into strips, or better, by making a network of rods over the roof with several connections to the earth at the corners, as I have before described.
Thus we see that the theory of lightning-rods, which appeared so simple in the time of Franklin, is to-day a very complicated one, and requires for its solution a very complete knowledge of the dynamics of electric currents. In the light of our present knowledge the frequent failure of the old system of rods is no mystery, for I doubt if there are a hundred buildings in the country properly protected from lightning. With our modern advances, perfect protection might be guaranteed in all cases, if expense were no object.
We have now considered the case of oscillations of electricity in a few cases, and can turn to that of steady currents. The closing of an electric circuit sends ethereal waves throughout space, but after the first shock the current flows steadily without producing any more waves. However, the properties of the space around the wire have been permanently altered, as we have already seen. Let us now study these properties more in detail. I have before me a wire in which I can produce a powerful current of electricity, and we have seen that the space around it has been so altered that a delicately suspended magnetic needle cannot remain quiet in all positions, but stretches itself at right angles to the wire, the north pole tending to revolve around it in one direction and the south pole in the other. This is a very old experiment, but we now regard it as evidence that the properties of the space around the wire have been altered rather than that the wire acts on the magnet from a distance.
Put, now, a plate of glass around the wire, the latter being vertical and the former with its plane horizontal, and pass a powerful current through the wire. On now sprinkling iron filings on the plate they arrange themselves in circles around the wire, and thus point out to us the celebrated lines of magnetic force of Faraday. Using two wires with currents in the same direction we get these other curves, and, testing the forces acting on the wire, we find that they are trying to move towards each other.
Again, pass the currents in the opposite directions and we get these other curves, and the currents repel each other. If we assume that the lines of force are like rubber bands which tend to shorten in the direction of their length and repel each other sideways, Faraday and Maxwell have shown that all magnetic attractions and repulsions are explained. The property which the presence of the electric current has conferred on the ether is then one by which it tends to shorten in one direction and spread out in the other two directions.
We have thus done away with action at a distance, and have accounted for magnetic attraction by a change in the intervening medium, as Faraday partly did almost fifty years ago. For this change in the surrounding medium is as much a past of the electric current as anything that goes on within the wire.
To illustrate this tension along the lines of force I have constructed this model, which represents the section of a coil of wire with a bar of iron within it. The rubber bands represent the lines of force which pass around the coil and through the iron bar, as they have an easier passage through the iron than the air. As we draw the bar down and let it go, you see that it is drawn upward and oscillates around its position of equilibrium until friction brings it to rest. Here, again, I have a coil of wire with an iron bar within it with one end resting on the floor. As we pass the current, and the lines of magnetic force form around the coil and pass through the iron, it is lifted upwards, although it weighs 24 lb., and oscillates around its position of equilibrium exactly the same as though it were sustained by rubber bands as its the model. The rubber bands in this case are invisible to our eye, but our mental vision pictures them as lines of magnetic force in the ether drawing the bar upward by their contractile force. This contractile force is no small quantity, as it may amount, in some cases, to one or even two hundred pounds to the square inch, and thus rivals the greatest pressure which we use in our steam-engines.
Thus the ether is, to-day, a much more important factor in science than the air we breathe. We are constantly surrounded by the two, and the presence of the air is manifest to us all we feel it, we hear by its aid, and we even see it under favourable circumstances, and the velocity of its motion as well as the amount of moisture it carries is a constant topic of conversation. The ether, on the other hand, eludes all our senses, and it is only with imagination, the eye of the mind, that its presence can be perceived. By its aid in conveying the vibrations we call light we are enabled to see the world around us; and by its other motions, which cause magnetism, the mariner steers his ship through the darkest night when the heavenly bodies are hid from view. When we speak in a telephone, the vibrations of the voice are carried forward to the distant point by waves in the ether, there again to be resolved into the sound waves of the air. When we use the electric light to illuminate our streets, it is the ether which conveys the energy along the wires as well as transmits it to our eye after it has assumed the form of light. We step upon an electric street-car and feel it driven forward with the power of many horses, and again it is the ether whose immense force we have brought under our control and made to serve our purpose--no longer a feeble, uncertain sort of medium, but a mighty power, extending throughout all space, and binding the whole universe together.
1 Based on reports in the (London) 'Electrician,' June 21 and 28 1889.
2 A striking illustration of this skin-deep penetration of high-voltage electricity was communicated by Lord Armstrong to Sir William Thomson (now Lord Kelvin) at the Newcastle meeting of the British Association in 1889. A bar of steel about a foot long, which Lord Armstrong was holding in his hand, was allowed accidentally to short circuit the two terminals of a dynamo giving an alternate current of 85 ampères, at a difference of potential of 103 volts. He instantly felt a sensation of burning and dropped the bar. His fingers were badly blistered, though on examining the bar a few seconds afterwards it was found to be quite cold. This proved that the action lay at the surface, and had not time to sensibly penetrate the substance of the bar. There were two little hollows burned out of the metal at the points where it touched the dynamo terminals.--J. J. F.
3 For Prof. Lodge's views see his paper, 'Jour. Inst. Elec. Engs.,' vol. xix. p. 352, and the very interesting discussion thereupon.--J. J. F.
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