CHAPTER II
ON FRICTIONAL AND CONTACT ELECTRICITY
The distinction generally found in textbooks on physics between the so-called "frictional" and "contact" electricity does not imply that there are two different kinds of electricity, but it refers to two different methods of producing electrification of bodies. Besides these two there are other methods, and some of them are of much greater practical importance. Those will be discussed in subsequent chapters; for the present we restrict the discussion to the two methods mentioned above.
The term "frictional electricity" indicates the process by which the electrification of a body is produced. If a stick of sealing-wax is rubbed with a flannel, both these bodies show electrification, but of opposite sign. We agree to call the electricity residing on the sealing-wax negative, and that on the flannel positive. Electricity may also be produced by rubbing a glass rod with a pad of leather, which has been covered with a mercury amalgam of zinc. In this case the glass rod shows positive electrification, and the pad negative. The old physics textbooks, therefore, also speak of a "vitreous" and a "resinous" electricity, meaning thereby respectively electric charges of positive and negative sign. The electrification is the result of friction between two different substances, one becoming positively and the other negatively charged. Probably any pair of bodies can thus be electrified, provided the necessary care is taken to prevent the accumulated charge leaking off. The process is not even restricted to solids; the friction between a solid and a gas also produces electrification. This fact is utilised in the Armstrong electric machine, where jets of steam are caused to flow past the spikes of a metal comb. By the friction of the steam against the surface of the metal the latter becomes electrified. It is also well known that the friction of the gas escaping through the valve of a balloon produces electrification of the envelope, and under certain circumstances so strong an electrification that a spark discharge may occur. This danger is avoided by the use of the ripping line on landing. The escape of gas then takes place through so large an opening that the velocity with which the gas passes the edges of the orifice is small, and the friction not sufficient to produce a sparking charge.
The friction between a pulley and its belt may in a dry atmosphere produce so strong a charge in the belt that sparks may be drawn from it. Such sparks are quite harmless to any person struck by them, but they may become a source of danger if inflammable substances are near. Thus in paper-making machinery, where the band of paper passes at high speed over hot metal rolls, it may become electrified to the extent of sparking and igniting itself. To avoid this danger it is necessary to fix spiked combs, which draw off the charge as soon as generated. In all these cases the electricity produced by friction is only an inconvenient by-product of some other operation; but if we wish to produce electricity for experimental work we may use special appliances based on the principle of electrification by friction. These are called "frictional machines." In substance they are nothing more than elaborations of the primitive glass rod and leather pad, so that the friction may take place under a suitable pressure and with sufficient speed. The machine is also fitted with spiked combs for taking off the negative charge from the pad and the positive from the glass, and generally there is some contrivance added for storing the charges, or one of them. Machines of this kind are very inefficient, and as they have within our generation been superseded by much more efficient machines working on a different principle, which are treated in the fourth chapter, we need not discuss them in detail.
The frictional machine was, however, up to the year 1789 the only practical means of producing such electrification as the physicist of those days required for his experiments. In that year there came a change. L. Galvani, Professor at the Bologna University, found that electric effects could be produced in animal tissue, if this were put into contact with two different metals, in his case copper and iron. His experiment with the frogs' legs is so well known that it would be wasting space to describe it here. Galvani looked for the cause of the phenomena observed in the tissue and not in the metals. In this he was mistaken. He assumed the existence of some mysterious "electric life force," and the name of "Galvanism" was given by the scientists of the time to this supposed force. This term has survived even to this day, though, except in some medical writings, rather in a metaphorical than a scientific sense.
Galvani's conception of an electric life force held the field for only a short time; it was proved to be a misconception by Alexander Volta, Professor at the Pavia University, who showed by a conclusive experiment that the cause of electrification does not reside in the animal tissue at all, but in the contact between the two different metals. He took discs of different metals, such as copper and iron or copper and zinc, and laid one on the other. The discs must be perfectly flat so as to present to each other even contact surfaces. Volta in his classic experiment found that such discs, if separated after having been in contact for ever so short a time, show signs of electrification; one being positively, the other negatively charged. In this experiment there is no question of any life force residing in animal tissue, for no such tissue is being used. The discs are simply laid one on the other, touched on the back, and then separated. Volta recognised that the cause of electrification was the contact pure and simple between the two dissimilar metals, and for this reason we may speak of "contact electricity" or "voltaic electricity" when we mean the kind of electrification first discovered by Volta.
Various theories have been set up to explain what may be termed the mechanism of this electrification. According to Helmholz, the molecules of a metal are endowed with the ability to attract and hold both electricities, but not with equal force. These molecular forces are different in different metals, and in consequence of these differences there takes place an actual separation between the two electricities at the boundary surface between the two metals. Other scientists (notably De la Rive) doubt the existence of such a molecular force in the metal itself, and look for the cause of electrification in the influence of an intervening link between the two metals, namely, the moisture of the atmosphere. They point out that even with the most accurate ground surfaces it is obviously impossible to make molecular contact between the two metals; that there must always be interposed a film of gases and vapours, and that it is by the intervention of this gaseous connecting link that the phenomenon called contact electricity takes place.
Whether the one set of theorists or the other have come near a true explanation, or whether both are mistaken, is not a matter which need concern us; the important fact is that electrification is produced by the contact between two metals, and that the intensity of their electrification, or the force by which the two electricities are separated across the boundary line of the two surfaces, does not depend on the extent of the surface of contact, but only on the quality of metals used in the combination. The force is greater with some combinations and smaller with others. By testing various combinations, it is thus possible to range all metals in a series, in which that metal which, combined with any other always shows a positive charge, stands at one end. This had already been done by Volta himself, who gave the series: Zinc—Lead—Tin—Iron—Copper—Silver—Gold. Zinc stands at the positive, and gold at the negative end of the series. This sequence has been verified by all later observers, who have also confirmed another observation originally made and published by Volta, namely, that the electric force between any two metals in the series is equal to the sum of the electric forces between all intermediate pairs. Thus, if in any arbitrary scale the electric force between zinc and lead is , and that between lead and copper , then the electric force between zinc and copper is .
The series given above ends with the most negative metal—gold; but Volta found that another substance, not a metal, but graphite, which is a special form of carbon, is still more negative than gold, and since Volta's time the series has been enlarged and extended by the addition of other metals and also sulphates and oxides, so that we must consider the phenomenon of electrification by contact to extend over a great variety of substances, and not to be restricted to a combination of metals.
Whether electrification is produced by friction or by contact, the process is in either case the separation of charges of electricity of opposite sign. We know that such charges attract each other, and that if accumulated on conductors sufficiently near, the conductors themselves will experience an attracting force. The tendency will be to bring the conductors together, and if they are held firmly in place, the tendency will be for the charges themselves to leave the conductors and unite. Whether they will actually do this depends on the distance between the nearest points of the conductors and the strength of the charges accumulated on them. Under certain conditions the force of attraction may be sufficiently great and the distance sufficiently small to cause electricity to leap across the intervening space, and then we have the familiar phenomenon of an electric spark.
The same phenomenon is observed in lightning, in which case the conductors may be two clouds charged with electricity, or a cloud and the earth. The force which in an electric machine causes the separation between positive and negative electricity is called the "electromotive force," and the practical unit in which the magnitude of electromotive force is expressed is called the "volt." To give the reader an idea of the size of this unit it may be mentioned that the electromotive force (or e.m.f.) with which electricity is caused to flow through an incandescent lamp is of the order of 100 to 250 volts, according to the type of lamp used. The most prevalent voltage employed for domestic lighting is 220 v. In comparison with this the e.m.f. of contact is very small, and that produced in a frictional machine is prodigiously large. The latter may easily reach tens of thousands or even hundreds of thousands of volts, whilst the e.m.f., under which lightning flashes occur, may be many millions of volts. Between the process of producing electrification by friction and producing it by contact, there is thus an enormous difference in degree, but no difference in kind, both processes being simply directed to the separation and isolation of charges of different sign.
If the positive and negative conductors of a frictional machine are connected by a wire, the charges rush along this wire to equalise each other, leaving both conductors without charge. We may imagine a simultaneous movement of positive and negative electricity along this wire in opposite directions, or we may imagine only the positive charge flowing along the wire in the direction of the negative conductor and spreading itself over its surface, and thereby neutralising the negative charge previously accumulated on it. What precisely takes place we do not know, but as a matter of convenience we assume that there is only one current, namely, that which flows from the positive to the negative conductor, much in the same way as water will always flow from the higher to the lower level.
Electricity, being an imponderable entity (in reality merely a form of energy), cannot be connected mentally with any conception of level, such as is legitimate in the study of the movement of heavy bodies. Nevertheless it is convenient to introduce a somewhat analogous conception to "high" and "low" when dealing with electrical problems, and this conception is that of "electric potential." Just as water tends to flow from the higher level to the lower level, so positive electricity has the tendency to flow from the conductor of higher to that of lower potential. The mechanical meaning of the term potential will be discussed in the following chapter; for the present it must suffice to note that as long as the two conductors are kept at a difference of potential by the working of the frictional machine, a current of electricity will flow from the positive to the negative conductor through the wire joining them.
The current obtainable from such a machine is exceedingly small, and any attempt to produce the electric currents required for lighting or other technical purposes by the use of a frictional machine is foredoomed to failure. Where currents of any magnitude are required, we must use other methods of producing electricity. These will be discussed subsequently, but for the present it is important to note that, apart from a question of degree, the frictional machine is an apparatus whereby electric currents may be generated.
How does the matter stand with regard to electrification by contact between solid bodies? Can we thereby also produce an electric current? We have seen that two metals in contact electrify each other. Using copper and zinc, the former becomes negatively and the latter positively electrified; that is to say, the zinc becomes the body of higher and the copper that of lower potential, and at first sight it might appear that by joining the back of the zinc disc to the back of the copper disc by a wire, we should get a current flowing along this wire from zinc to copper. This is, however, not the case.
Whatever the material of the joining wire may be, it must fall somewhere into the series of contact e.m.f., and be subjected to the law that the sum of its potential differences to zinc on the one side and to copper on the other side is equal to the potential difference between zinc and copper. We thus have a perfect balance of e.m.f.'s set up by the direct contact between the two discs and the indirect contact via the joining wire. Since the e.m.f.'s are in equilibrium, no current can flow. If it were possible to upset this equilibrium on one side or the other, then we could produce a current, and that is actually done by an arrangement of substances, some of which fall outside the series of contact e.m.f.'s. Such arrangements are called "voltaic cells." A familiar example is the so-called Leclanché cell (named after its inventor), which is found in almost every household for the working of electric bells.
Before entering on a study of voltaic cells it will be convenient to amplify the series on p. 40 by the definite statement of the e.m.f. to be obtained with any combination of the metals. The figures in the following table represent experimental results obtained by Ayrton and Perry, and recorded in Whetham's Practical Electricity—
Table of Contact E.M.F. in Volts
(Zinc is positive in relation to all the other substances given in this table.)
| Substance. | Zinc. | Lead. | Tin. | Iron. | Copper. | Platinum. | Carbon. | |||||||
| Zinc | 0 | 0.210 | 0.279 | 0.592 | 0.738 | 0.976 | 1.089 | |||||||
| Lead | -0.210 | 0 | 0.069 | 0.382 | 0.528 | 0.766 | 0.879 | |||||||
| Tin | -0.279 | -0.069 | 0 | 0.313 | 0.459 | 0.697 | 0.810 | |||||||
| Iron | -0.592 | -0.382 | -0.313 | 0 | 0.146 | 0.384 | 0.497 | |||||||
| Copper | -0.738 | -0.528 | -0.495 | -0.146 | 0 | 0.238 | 0.351 | |||||||
| Platinum | -0.976 | -0.766 | -0.697 | -0.384 | -0.238 | 0 | 0.113 | |||||||
| Carbon | -1.089 | -0.379 | -0.810 | -0.497 | -0.351 | -0.118 | 0 |
We have seen that no current due to contact e.m.f. can be produced in a circuit the members of which all belong to a series of contact e.m.f., and which therefore fall under the law that the potential difference between any two is equal to the sum of the potential differences of the intervening pairs. Whichever way we go round such a circuit the total e.m.f. is always zero. To get an e.m.f., and therefore a current in the circuit, we must find some conducting material which falls outside the series in the sense that it does not obey the law just stated. If the continuity of metallic contacts is interrupted by the interposition of such a material, then there will be no complete equilibrium, but a balance of e.m.f. in a definite direction and a current will result. Water is such a material; it becomes strongly positive when in contact with any of the substances given in the table, but the difference of e.m.f. of the two combinations, water-zinc and water-copper, is not equal to that of the combination copper-zinc.
Assume for the moment that the difference is zero, or, in other words, that water
Fig. 1.
is quite inert as regards contact e.m.f. and simply acts as a conductor. This is not actually the case, but a convenient assumption for the purpose of explaining the way a cell may give an e.m.f. in an external circuit. Let, in Fig. 1, Zn and Cu be a zinc and copper plate respectively, and let to these plates be fastened strips of copper for the attachment of the terminals A and B. The plates are not directly in contact, but are placed in a vessel filled with water. As we are only dealing with potential differences, we may arbitrarily fix the potential of one terminal at zero. Let this be the end of the copper strip soldered at a to the zinc plate. Then the potential of the zinc plate, which is due to the contact e.m.f. of the junction a where the copper strip is soldered to it, will by the table on p. 47 be 0.738 volts. Since by hypothesis the water is inert both as regards the zinc and the copper, this will also be the potential of the copper. The junction at b cannot alter this value, since at that place two equal metals are in contact. The potential at B is therefore also 0.738 volts, and on joining A with B by a wire a current will flow. Now let us replace the water by a dilute solution of sulphuric acid. The difference of contact e.m.f. of this liquid in relation to zinc on the one hand and copper on the other is about one-third of a volt, and this difference acts in the same sense as the contact e.m.f. at a. The result is that the potential of the copper plate, and therefore also of its terminal B, has now been increased to a little over one volt. Here we have a combination of substances, which, by virtue of contact e.m.f., are causing a current to flow. In this primitive form the arrangement is, however, very imperfect.
As soon as the current flows, there come into play some secondary actions, which cause the contact e.m.f. between zinc and liquid, which is in a forward direction, to decrease, and that between liquid and copper, which is in a backward direction, to increase. The contact e.m.f. causes the current to flow, but as soon as the current flows, this current itself reduces the contact e.m.f. This reaction may be illustrated in a homely way by saying appetite causes a man to eat, but when eating he loses his appetite.
This interdependence between cause and effect is observable in all physical processes, and in its bearing upon the relation between electric currents and mechanical forces it has been formulated by Lenz, and is known as Lenz's law. Here we have to do not with mechanical, but with chemical forces. The current, in passing through the liquid, decomposes it, sending oxygen to the zinc, which is dissolved, and hydrogen to the copper, where it forms a coating and introduces an additional counter e.m.f. of contact. This process is technically termed "polarisation" of a cell, and the ingenuity of inventors has been and is even at the present day exercised in finding means to avoid or at least reduce the effect of polarisation.
The first and completely successful attempt in this direction has been made by Daniell, in 1836. He recognised that the cure for polarisation lay in preventing any hydrogen being liberated and carried to the copper plate. If the liquid in the immediate vicinity of the copper plate contained a copper salt, it would not be hydrogen molecules, but copper molecules that are precipitated on the copper plate, and this could, of course, not alter the original condition of the cell. He used, therefore, a solution of sulphate of copper as the liquid into which the copper plate is immersed. But now arises another difficulty. We must not let the copper sulphate come into contact with the zinc, for this would not give the desired e.m.f., and it would also, by reason of the dissolution of the zinc, very quickly spoil the solution. It is thus necessary to still use dilute sulphuric acid as the liquid into which the zinc is immersed, and at the same time anything like a mixing of the two liquids must be avoided. This object is attained by the employment of a porous pot for the separation of the two liquids. The porous pot forms the inner vessel into which the acid and zinc are placed, whilst an outer vessel is provided for the reception of the copper plate, and the solution of copper sulphate. It is advisable to amalgamate the zinc with mercury so as to protect it against attack by the acid when the cell is not working. When it is working no protection is possible, for the electrochemical action must be going on as long as the current flows. By this action oxygen is carried to the zinc, and this is thereby dissolved, forming with the sulphuric acid zinc sulphate. Thus the electrical energy given by the cell to the external circuit is obtained at the cost of the chemical energy liberated in the oxydation of the zinc and its conversion into sulphate. The e.m.f . of the Daniell cell is quite constant; it is a little over one volt.
Since Daniell's time many types of depolarising cells have been invented, zinc being generally one of the metals employed. The current passes from the zinc through the liquid to the other plate, which may be of copper, as in the Daniell and Meidinger cell, or of platinum as in Grove's, or of carbon as in Bunsen's and others. Since the current issues from the cell at the platinum or carbon plate, the terminal in connection with this plate is called the positive pole of the cell, the zinc terminal being the negative pole.
One of the most largely used types of cell with zinc-carbon electrodes is that designed by Leclanché. The liquid used in this cell is a dilute solution of salammonia, and the polarisation of the carbon is counteracted by the employment of a metallic oxide in contact with it. The carbon plate is placed into a porous pot and packed round tightly with a mixture of granular gas coke and manganese peroxide. This substance is a powerful oxydising agent; it gets hold of the molecules of hydrogen on their way to the carbon electrode, and thus prevents them settling there and causing a back e.m.f. of polarisation. This chemical action can, however, only go on at a moderate rate, so that the Leclanché cell is mostly used where weak and intermittent currents are required, as for instance, in the working of electric bells. If the cell is worked too hard, the chemical action, whereby polarisation is rendered innocuous, cannot keep pace with the rate at which hydrogen is carried to the carbon plate, and the e.m.f. of the cell, which under normal conditions is about 1.4 volts, drops to a much smaller figure. If left standing idle a little while, the cell recovers and its e.m.f. rises again to 1.4 volts. It will be obvious that by joining up in the same sense a sufficient number of Daniell cells, or cells of any other type, any desired voltage may be obtained between the ends of the series of cells.
A special type of cell is the so called accumulator or storage cell, in which both electrodes are lead and some oxide of lead. This is a so-called reversible cell. On forcing a current through in one direction, the oxide on the plate where the current enters the electrolyte (dilute sulphuric acid) is reduced, and the other electrode becomes more highly oxidised. Thus the cell is charged. If then the cell is connected to any working circuit, it gives a current in the reverse direction; the previously strongly oxidised electrode being reduced and the other becoming more oxidised, the cell discharges. These lead accumulator cells are made up into storage batteries, which are extensively used in electricity works.