Lives of the electricians

by William T. Jeans

Professor A. de la Rue stated in 1843, before Professor Tyndall had begun his scientific studies, that the study of electricity was always a favourite and popular study in England, and as evidence of that observation he added that Professor Faraday had delivered in London lectures on electricity at the Royal Institution, to which resorted in crowds not only men of the world and elegant ladies, who came in great numbers to admire the graces and enjoy the charm which the amiable professor so well knew how to diffuse over his teaching, but also savants who always found something new to acquire from the interesting views of the learned philosopher. These words might with equal propriety be applied to the lectures of Professor Tyndall. During his reign the Royal Institution made marked progress in popularity and usefulness. According to his own statement, the main object of its existence is that of a school of research and discovery; and during the whole time he has been there no manager or member of the Institution ever interfered with his researches, though a bye-law gave them power to do so. The salient features of his researches have already been described; but only those who have had the privilege of hearing the Professor’s own descriptions, and seen his simple and beautiful experiments illustrating the subtle laws of matter, can adequately appreciate the charm with which he invests scientific subjects. It is not an unusual occurrence for the theatre to be full of people nearly an hour before the lecture begins, and whether addressing an audience of young or old people, he rivets attention by his easy, lucid, and fascinating exposition and illustrations of the science of electricity, heat, light, and sound.

As a specimen of the descriptive power with which he can impart interest to a subject generally regarded as unattractive, take the following exposition of the development of electricity:—“Volta found that by placing different metals in contact with each other, and separating every two pairs of metals by what he called a ‘moist conductor,’ he obtained the development of electricity. He imagined that the source of power was simply the contact of the two metals that he employed; he regarded the moist conductor as a neutral body; and his theory was called, in consequence of this view, the ‘contact theory.’ He was perfectly correct in affirming that the contact of different metals produces electricity; one of the metals in contact being positive, and the other being negative. The voltaic current was capable of producing light and heat; but light and heat require the expenditure of power to produce them; and it was shown by Roget that if Volta’s conception were correct, it would be tantamount to the production of a perpetual motion; if the simple contact of metals produced an unfailing source of electricity, it would be the creation of power out of nothing. Here Volta failed. Afterward he devised an instrument which showed the conversion of mechanical power into electricity, and thus into heat and light. That instrument he called the electrophorus, and it furnishes perhaps the simplest means of showing the conversion of mechanical power into electricity, and thence into heat and light. Volta himself was not aware of the doctrines which we now apply to his discoveries. I will go through the form of Volta’s experiment. I have here a piece of vulcanised indiarubber, and I would first remark that when I place a sheet of tin with an insulating handle upon the table and lift it, I simply overcome the gravity of the tin; but if, after having whisked a sheet of vulcanised indiarubber with a fox’s brush, I place the plate upon it, I find that on lifting it something more than the weight of the plate is to be overcome. That plate now is in a different condition from its former one. It is now electrified, and if I bring my knuckle near it I receive an electric spark. What I want to make clear is this: that there is, first of all, the expenditure of an extra amount of mechanical force in order to lift the sheet of tin; that, by the lifting of the tin, you liberate electricity upon its surface; and that then, if you bring your knuckle near it, you receive an electric spark. There is, therefore, first of all, an expenditure of mechanical power in lifting the sheet of tin; then an intermediate stage when the tin is electrified; and finally, the passage through that electric stage into heat. So that you have mechanical power, electricity, and heat; mechanical power and heat being the two extremes of the circuit.

“When you have electricity developed, the connection of heat and light is necessarily accompanied by resistance to the passage of the electricity. The action of lightning conductors, for example, is entirely dependent upon that fact. The chimneys that the conductors protect offer resistance to the passage of the discharge, and therefore would be destroyed by that discharge; but the conductor offering small resistance, the current passes through it without any disruptive action.

“I will explain the principles of an ordinary Grove’s battery, in order to give a better idea of what internal and external resistances there are in the current. In a Grove’s battery there are two metals, zinc and platinum. They are in contact with each other. There are also two liquids, nitric acid and dilute sulphuric acid. If I connect by a wire one end or pole of the battery with the other, I, being close at hand, can see a small spark. There is now flowing through that connecting wire what we call an electric current, which passes from one end of the battery through the wire to the other end. When there is very little resistance offered to the passage of the current, there is no sensible heat developed; but if I sever the wire in the middle and unite the ends by a thin platinum wire, the thin platinum wire introduced into the circuit is first raised to incandescence and then fused. It is because of the resistance that it offers that we see the incandescence of the wire.

“The source of power in this battery is the combustion, for it is to all intents and purposes combustion of the metal zinc. When we connect the two poles of that battery by a thick wire we have no sensible external heat produced. The heat due to the combustion of the zinc is liberated wholly in the cells of the battery itself. That quantity of heat, as is very well known, is the amount developed by the solution or oxidation of zinc in dilute sulphuric acid. Supposing that we allowed the current to pass through the thick wire until a certain definite weight of zinc was dissolved in the battery, that would produce in the cells of the battery a perfectly definite amount of heat. Let us compare that amount of heat with the amount produced in the battery when we introduce the thin platinum wire. In the one case we have no external heat, and in the other we have. The great law which regulates these transactions is this: that the sum of the internal and the external heats is a constant quantity; so that when the platinum wire was ignited we had less heat developed in the battery than before. The zinc in the battery is burned as fuel upon a hearth; the heat, however, being developed either upon the hearth itself or at any distance from it.

“As a primary source of electricity here is the combustion of a metal, the voltaic battery is not an economical source of power for producing electric light. Had it been so we should have employed the electric light long before the present time. Davy, seventy years ago, made most important experiments upon the light and heat of the voltaic circuit, but the reason why it was not applied previously is simply that zinc is an exceedingly expensive fuel. That stopped the economical application of the electric light to the purposes of public lighting.

“If we burnt the zinc in the open air instead of in the battery there would be a considerable amount of heat and light produced. To burn it in the acid fluid of the battery, afterwards converting it into heat and light, is only another mode of burning it: both are due to the same combustion.

“In the year 1820 Arago discovered that when he carried an electric current parallel to a magnetic needle, he deflected the needle to the right or to the left, as the case may be. Soon afterwards one of the greatest geniuses that ever lived, Ampère, within eight or ten days of the description of [OE]rsted’s discovery before the Academy of Sciences of Paris, enriched this field by a sudden burst of new discoveries and experiments. To Ampère we are indebted for our knowledge of the action of electric currents one upon another. For instance, if I suspend two flat coils in the presence of each other, it is easy to send an electric current in the same direction through both. The consequence of that would be an immediate attraction of the two coils for each other. It would be also easy to send currents in opposite directions, and the immediate consequence of that would be repulsion. If, having sent an electric current through one of these coils, a magnet is brought to bear upon it, the coil and the magnet interact almost like two magnets. The great law established by Ampère was that currents flowing in the same direction attract each other, whilst currents flowing in opposite directions repel each other. To show the interaction of magnets and currents, and to illustrate the simulation, if I may use the term, of magnetism by electricity, Ampère, by an extremely ingenious device, suspended spiral wires, and proved that when an electric current is sent through such a wire, it behaves, to all intents and purposes, like a magnet; it will set like a magnetic needle in the magnetic meridian. It was Ampère who first of all established the interaction of electric currents amongst themselves, and also between electric currents and magnets.

“Arago was engaged at the same time in joint work with Ampère. Perhaps one or two further illustrations might be given. Here we have a piece of copper wire. At the present moment there is no action whatever of that wire upon iron filings; the copper wire has no magnetic power whatever. But I send what for want of a better name, we call an electric current, through the wire, and then the iron filings crowd round the wire. If I break the circuit, the magic entirely disappears. This is one of the effects that enables us to see that a current is passing through the wire. Arago, who noticed this, went further and showed that, when you coil a wire round a piece of iron, the piece of iron is rendered strongly magnetic by the passage of the current through the wire.”

It is, however, as an experimentalist that Professor Tyndall excels, especially in illustrating by experiments the effects of electricity and magnetism. He was the first to show publicly the elongation of a solid bar of iron by magnetising it. He had a small mirror so connected with the end of a bar of iron two feet long that it reflected a long beam of light on a screen, and the beam moved on the screen as the bar of iron was lengthened or shortened. When the iron was magnetised by electricity from a battery the mirror showed a lengthening movement on the screen; and he explained that the bar being composed of irregular crystalline granules, the magnetism tended to set the longest dimensions of the granules lengthwise, or parallel to the flow of the current. Mr. Joule who discovered this lengthening effect of magnetism, found that a bar of soft iron was by this means extended one 720,000th of its length; and in later years Professor Hughes demonstrated the mechanical theory of magnetism, which, like the mechanical theory of heat, attributes such phenomena to a simple mechanical motion of the molecules of matter. Numerous researches and experiments led him to the conclusion that each molecule of a piece of iron, as well as the atoms of all matter, solid, liquid, and gaseous, is a separate and independent magnet, that each molecule can be rotated upon its axis by magnetism and electricity, and that the inherent polarity or magnetism of each molecule is a constant quantity like gravity.