not three straight pieces. This peculiarity of the iron line was very strikingly shown by forming circuits of iron with the alloys PtIr or PtCu. Such circuits exhibit two or even three neutral points (see fig. 55). Another very elegant

Pa

Au

Experi

lines corresponding to two metals, say Cu and Fe (of which the former is above the latter in the thermoelectric series at ordinary temperatures), and if AB, A'B' be the ordinates corresponding to 0 and 6, then the electromotive force in a circuit of the two metals whose junctions are at the temperatures and ', tending to send a current from Cu to Fe across the hotter junction, is represented by the area ABB'A'. The Peltier effects at the two junctions are represented by the rectangles ABba and A'B'b'a', and the Thomson effects, in the Cu and Fe respectively, by AA'DC and BB'GF, or by AA'a'a and BB'b'b, which are equal to these At N, where the lines intersect, the Peltier effect vanishes. N therefore is the neutral point; and, if the higher temperature lie beyond it, the electromotive force must be found by taking the difference of the areas NA'B' and NAB, and so on. All the phenomena of inversion may be studied by means of this diagram, and the reader will find it by far the best means for fixing the facts in his memory.

For several years back Tait1 and his pupils have ments of been engaged in verifying the consequences of this con

Tait, &c. jecture; and it has been shown, first, for temperatures

within the range of mercury thermometers, and latterly for temperatures considerably beyond this range, that the hypothesis accords with experience. The methods cmployed by Tait in his experiments at high temperatures are of great interest and importance. One of these was to construct a curve whose ordinate and abscissa aro the simultaneous readings of two thermoelectric circuits whose hot and whose cold junctions are kept at common temperatures. It is a consequence of the foregoing assumption that the curve thus obtained ought to be a parabola. Very good parabolas were in many cases obtained.

In some

cases, however, the curves, so far from being parabolas, were actually curves having points of contrary flexure. This anomaly led Tait to the discovery of the astonishing fact that the Thomson effect in iron changes its sign certainly once at a temperature nçar low red heat, if not a second time near the melting point. It was found that the inflected curves could be represented by piecing together different parabolas. Hence the line for iron in the

Trans. R. S. E., 1873.

Fig. 55.

method of verification consisted in using along with an iron wire a multiple wire of Au and Pd, the resistances of whose branches could be modified at will. It is easy enough to show that the line for the Au-Pd wire is a straight line, passing through the neutral point of Au and Pd, and such that it divides the part of an ordinate lying between the Au and Pd lines in the ratio of the respective conductivities of the Au and Pd branches. Thus, by increasing ratios of the conductivities of the Pd and Au branches from 0 up to ∞o, we can make the Au-Pd line sweep through the whole of the space between Au and Pd (fig. 55), and thus explore the part of the Fe line lying in the space. We get in this way first one neutral point, then two, then one, and then none in our Fe, Au-Pd

circuit.

from

son

alon

Tait has pointed out that, by using PtIr and Fe, and Curr keeping the hot and cold junctions at the two neutral Tho temperatures, we get a current maintained solely by the excess of the heat absorbed in the hotter iron over that The electromotive force is repredeveloped in the colder. sented by the area inclosed by the part of the zigzag on the Fe line cut off by the PtIr line (fig. 55). A similar case of thermoelectromotive force without Peltier effects may be obtained with three metals, such as Fe, Cd, Cu, electromotive force in this case is represented by the whose neutral points lie within reasonable limits. The triangle between the three lines.

We subjoin a table, calculated by Professor Everett from Tait's Tail diagram. The thermoelectric power is given in electromagnetic res (C.G.S.) units, in terms of the temperature () in centigrade. degrees, by means of the formula a+ßt, where a and 8 have the tabulated values :

of absolute measurements of the Peltier effect with Tait's theory; but, unfortunately, no data that we know of are available for the purpose. It is absolutely necessary for this purpose to have heat measurements and determinations of the lines of the metals in the same specimens. The data of Edlund1 and Le Roux are quite useless for such a purpose. One result of Le Roux's is, however, interesting. He finds for the amount of heat developed at the junction BiCu, the values 3.09 and 3.95 at 25° C. and 100° C. respectively. Since the neutral temperature of BiCu is very high, the Peltier effect ought, according to Tait's theory, to vary as the absolute temperature. The absolute temperatures corresponding to 25° C and 100° C. are 298 and 373°, and we have 3.95÷3.09–1.278, while 373÷298-1252; the agreement between these numbers bears out the theory so far.

General Considerations regarding the Seat of Electromoat of tive Force.-Before proceeding to notice the remaining cases of the origin of electromotive force, in which the phenomena are more complicated, and the experimental conditions less understood, it may be well to call attention to & principle that appears to hold in most of the cases already examined. In most of these cases the seat of the electromotive force appears to be at the places where energy is either taken in or given out in the circuit.

It is very natural to ask ourselves what the consequences would be if we applied this principle to the voltaic circuit. It would probably be admitted by most that the energy in the voltaic circuit is taken in mainly at the surface of the electropositive metal. This admission, taken in conjunction with the general principle above stated, leads us to the conclusion that the electromotive force resides mainly at the surface of the electropositive metal. The absorption or evolution of energy at the junction of the dissimilar metals is quite insignificant, and we should, on the same view, deny that any considerable part of the electromotive force resides there.

This view appears to be at variance with the theory of metallic contact, as now held by Sir William Thomson and others; and the burden of explaining the experiments made by him and others on the contact force of Volta is doubtless thrown on those who adopt this view. The position of such would very likely be that there is an uneliminated source of uncertainty in all these experiments (see above, p. 85). On the other hand, those who adopt the contact force of Volta at the junction of copper and zinc as the main part of the electromotive force of Daniell's element are under the necessity of distinguishing this from the electromotive force corresponding to the Peltier effect, which must be a distinct effect, since it is but a very small fraction of that of a Daniell's cell.

We are, however, so very ignorant of the nature of the motion which is the essence of the electric current that the very form in which we have put the question may be misleading. If this motion be in the surrounding medium, as there is great reason to believe it to be, it would not be surprising to find that speculations as to the exact locality of the electromotive force in the circuit were utterly wide of the mark. The very language which we use implies a certain mode of analysing the problem which may be altogether wrong. The only thing of which we can as yet be sure is that the mathematical equations deduced

1 Wied. Gale., Bd. i. § 694.

* Since the above was written further experimental evidence in suppart of the theory has appeared. See Naccari and Bellati, Atti del R. Ist. Veneto di Sc. Litt. ed Arti, November 1877.

Maxwell, vol. i. § 249. By "being taken in," in the case of heat for instanee, is meant "disappearing as heat and appearing as electrokinetic energy." In a thermoelectric circuit this transformation occurs wherever there is Peltier or Thomson effect.

Maxwell, L.c.

from Ohm's law and other proximate principles are in exact accordance with experiment.

Pyroelectricity. Some account of this interesting subject has Pyroalready been given in the Historical Sketch at the beginning of this elec tricity sions of those who have recently investigated the matter. It seems article. It will be well, Lowever, to state here some of the conclunow to be settled that it is not merely high or low temperature, but change of temperature, which gives rise to the electrical phenomena maybe described thus. One end A of the crystal is distinguishable of pyroelectric crystals. The properties exhibited by tourmaline from the other end B by the dissymmetry of the crystalline form. A is called the analogous pole of the crystal, and B the antilogous pole. When the temperature of the crystal is increasing uniformly throughout, the analagous pole is positively electrified and the antilogous pole negatively electrified. When the temperature is decreasing uniformly throughout, the analogous pole is negative and the antilogous pole positive. This law was originally dis covered by Canton, but it seems to have been lost sight of again Canton and rediscovered both by Bergman and by Wilcke in 1766. When the Wilcke. temperature is uniform, the positive and negative regions are sym- man metrically distributed about the central zone of the crystal, which is neutral. If the ends be unequally heated, this symmetry no longer obtains. It must not be forgotten that complications may arise from the crystal becoming electrical as a whole by friction, usually positive, like most other vitreous bodies.

Berg

Thom

son's

Gaugain made a series of interesting experiments on the electrical properties of tourmaline, and concluded that a tourmaline Gaugain. whose temperature is varying may be compared to a voltaic battery of great internal resistance, consisting of an infinite number of cells, each of infinitely small electromotive force; so that the electromotive force is proportional to the length of the tourmaline, and its internal resistances is proportional to the section inversely and to the length directly. He also concluded that the amount of elec. tricity furnished by a tourmaline, while its temperature varies either way between two given temperatures, is always the same. In order to explain the properties of the tourmaline, it has been supposed that the crystal is naturally in a state of electrical pola rization, like that assumed by Maxwell in a medium; under the influence of electromotive force, or more nearly (since no sustaining theory. force having an external origin is supposed) like that of a permanent magnet. The intensity of this polarization is supposed to be a function of the temperature. Supposing the tourmaline to remain for some time at the same temperature, a surface layer of electricity would be formed, which would completely mask the electrical polarization of the crystal, inasmuch as it would destroy all external electrical action. This neutralization would be instantly effected by running the crystal through the flame of a lamp. If, however, the temperature increase, then the polarization will, let us say, increase, so that the surface electrification no longer balances it. We shall thus get polar electrical properties of a certain kind. If the temperature decrease, the polarization will decr 'se, and we shall thus get polar properties of the opposite kind.

In many pyroelectric crystals there are more than one electric axis, so that we have several analogous and corresponding antilo gous poles. An enumeration of the various crystals in which pyroelectric properties have been found, and a discussion of the peculiarities in their crystalline form, belongs more properly to the science of Mineralogy. Much has been done in this department by Köhler, Gustav Rose and Rices, and Hankel. 10 For some very interesting researches by Friedel see Annales de Chimie et de Physique,

1869.

Contact

Frictional Electricity.-In accordance with the general principle laid down at the beginning of this section, we should expect to find an electromotive force at the surface which separates two different non-conducting media, just as we have found it at the boundary of two different conducting media. The effect of such a contact forco of nonwould be very different however in the former of these cases, from conduc what we have seen it to be in the latter. In the case of non-con- tors. ductors the electricity cannot leave the surface of separation, but will simply accumulate on the two sides of it, till the force arising from electrical separation is equal to the contact force. On separating the bodies, in certain cases, we may carry away with us these surface layers of electricity, and it is an obvious consequence of our principles that the electrifications of parts of the two bodies that have been in contact must be equal and opposite. While the bodies are in contact the difference of potential between the layers of electricity corresponding to very considerable surface density may be very small, just as in Volte's condensing electroscope (see above, p. 34); but when we separate the bodies work is done against the electrical attractions, and the potential increases enormously. Mascart, t. ii.

Phil. Trans., 1759.

fric

These hypothetical results tally very well with the electrical phenonena observed when non-conducting bodies are lightly rubbed together; and the above is nearly the explanation that most physicists of the present day would probably give (if they gave any) of what is called the "frictional generation of electricity." All experimenters are agreed that equal quantities of positive and negative electricity appear in this case as in every other case of electrical separation; an experiment to prove the contrary would have to be very demonstrative indeed before it would now be accepted as conclusive. A single case of exception would revolu tionize our fundamental ideas completely. The reader should consult on this point Faraday's Experimental Rescarches, series xi. Tii.

The other consequences of our hypothesis are by no means so firmly established. One of these is that we ought to be able to arrange non-conducting bodies in a series such that any body rubbed with one below it in the series becomes positive, and rubbed by one above it negative.

Many electricians have attempted to establish such electromotive series, but the experimental conditions (see the admirable remarks of Riess, Reibungselectricitat, § 907) are so complicated that nothing absolute has been attained. Yet it would appear that, if we could make sure that we were always dealing with definite materials under definite surface conditions, electromotive series could be constructed in which every different body would have a fixed position. As it is, the body bearing the same name in the lists of different experimenters was in all probability not exactly of the same material in all cases, and (we might say certainly) was not under the same surface conditions. We refer the reader to Riess (1.c.) for an admirable résumé of the work of different electricians in this department. Mascart has given a very interesting account of the matter (t. ii. § 834, &c.) from a more modern point of view. From these sources, together with indications in Young's Lectures on Natural Philosophy, the reader will be able to follow up the literature of this somewhat uninviting department of electricity.

We give two instances of frictional electromotive series which tional may be useful in giving the reader a general idea how different series bodies stand. Wilcke.

The following is Wilcke's series1 (1758):-Glass, woollen cloth, feathers, wood, paper, shellac, white wax, ground glass, lead, sulphur, metals.

Faraday gives-cat and bear skin, flannel, ivory, feathers, rock Faraday. crystal, flint glass, cotton, linen, white silk, the hand, wood, shellac, metals (iron, copper, brass, tin, silver, and platinum), sulphur.

experiments.

To which Riess adds (in order) the highly negative bodies-guttapercha, electrical paper, collodion, gun cotton.

Considered as evidence for the contact hypothesis, the experiments of Peclet seem to be important. He used an apparatus which was Peclet's virtually a Nairne's machine (see below, p. 101), in which the rubber could be varied at will. His general conclusions are quite in accordance with the contact theory. He found, for instance, that for the great majority of materials the quantity of electricity generated was independent of the pressure and of the breadth of the rubber, and varied as the angular velocity of the cylinder, and it even appeared to be the same for rolling friction as for sliding friction, so long as the material of the rubber was unchanged.

of conductor with non

Besides the case of two non-conductors, we might consider the case of a conductor and a non-conductor in contact. Much of what Contact has just been said would apply to this case also, an excellent example of which is furnished by a frictional electrical machine of the ordinary construction when the cushions are well furnished with Conduc amalgam. This is the place to give a short account of these timehonoured pieces of electrical apparatus. For a history of them we cannot do better than refer to Mascart" (1.c.), who has devoted much attention to the theory as well as the history of electrical machines in general.

tor.

Fic

na

A very common form of machine, called Ramsden's, is pictured ti nal in fig. 56. It consists, like all other frictional machines, essentially of three parts-(1) the rubbed or moving body, (2) the chines. rubbers, and (3) the collectors and prime conductors. In the present instance the rubbed body is a disc of glass, which can be turned about a horizontal axis by means of a suitable handle. The efficiency of the machine depends very much on the quality of the glass of which the disc is made. According to Mascart, glass of old manufacture is superior to the more modern specimens, owing to the smaller proportion of alkali ip the former; it appears, however, that the disc improves in most cases with age and use. Many Exp. Res., 2141.

According to Riess, the earliest.

The parchment-like paper obtained by treating ordinary paper With concentrated sulphuric acid.

rotation.

That is, the dimension of the rubber perpendicular to the axis of A few notices of the earlier machines will be found in the Historical Sketch.

other materials have been proposed to replace glass, which is somewhat costly when large discs are required. Ebonite has been tried a good deal of late, and has great advantages so far as its electrical properties are concerned; but it has the disadvantage that it warps very readily. if heated incautiously, and its surface will not keep good for any length of time. Owing to decomposition under the action of light, a layer of sulphuric acid forms on the surface, after which it is very difficult to restore the electrical virtue so remarkable in the new material, although washing with hot water or immersion in a blast of steam are said to be effective in some degree.

The rubbers consist of two rectangular pieces of wood, hinged to supports attached to the framework of the machine, and fitted with springs and screws, SO that they can be made to clip the plate with any required pressure, The rubbing surfaces are usually formed of leather, stretched as smooth and flat as possible (oiled silk is sometimes used, but it is not so durable). Before the leather cushions are fit for use, they must be carefully coated with amalgam. The amalgam most commonly used is Kienmayer's, which is a composition of two parts of mercury with one of zinc and one of tin. A great variety of different compounds of this kind have been used by different electricians, bisulphide of tin being a general favourite. The amalgam must be powdered as finely as possible, all grit being carefully removed. The cushions are then to be lightly smeared with lard, and worked together till the surface is very smooth and the greasiness almost gone; then the amalgam is to be carefully spread over them, and the surfaces again worked together till a uniform metallic surface is attained, they are then ready for use. The amalgam aids the action of the machine in two ways,-first, by presenting a surface which is highly negative to glass; secondly, by allowing the negative electricity evolved by friction to flow away without hindrance from the points of contact. In order to secure the second of these advantages still more perfectly, the cushions should be carefully connected by strips of tinfoil, or otherwise, with knobs, which can be put to earth during the action

FIG. 56.-Ramsden's electrical machine.

of the machine.

The collectors are two stout metal forks bestriding the glass disc at the ends of a horizontal diameter. They are armed, on the sides next the glass, with rows of sharp points, which extend across the rubbed part of the disc. The prime conductor in the specimen we are describing forms a metal arch rising over the framework of the machine, and insulated from the sole by two glass pillars. Various forms are given to this part of the machine, according to the fancy or convenience of the experimenter. thing to be seen to is, that there be no salient points on it which might facilitate the dissipation of electricity by brush, convective, or spark discharge,

One important

After what has been said, the action of the machine requires little explanation. The disc, electrified positively by contact with the amalgam, carries away a positive charge, whose potential rises rapidly as it leaves the cushion,-so high, in fact, that there is a tendency to discharge to the air, which is prevented by covering the excited parts of the disc by pieces of oiled silk. When the highly charged glass comes opposite the points of the collector, owing to the inductive action, negative electricity issues from the points and neutralizes the charged plate, which at this point is vir tually inside a closed conductor. The result of this is that the prime conductor becomes positively charged. The neutralized parts of the disc now pass on to be rubbed by the other cushion, and so on. The electricity goes on accumplating in the prime conductor until the potential is so great that discharge by surface conduction, or by spark, takes place between the collectors and the cushion, or between the collectors and the axis.

If it is desired to obtain negative electricity from a machine with a glass disc, we have simply to connect the prime conductor to earth, insulate the cushions, and collect the electricity from them. We have said that there is a limit to the potential to which the

Mancart, 2.c.

Nairne's

charge on the prime conductor can be raised. We can never get a longer spark from the machine than the length of the interval between the collector and the cushion or the axis, as the case may be. The limiting potential can, however, be increased by insulating the axis of the machine, or making the axis itself wholly or partially of insulating material, and by using only one rubber and ne collector, and placing them at the extremities of a diameter. The machine of Le Roy, often called Winter's machine (fig. 57), is

FIG. 57. Le Roy's machine. Constructed on this pattern. We get, of course, cæteris paribus, only half as much electricity per revolution with a machine of this kind as with Ramsden's; but the spark is longer, in consequence of the greater insulation between the cushion (A) and the collector (B). The cylinder machine, also called Nairne's machine, was one of achine. the first machines in which all the essential parts of the modern frictional machine appeared. It consists of a glass cylinder, which can be turned about a horizontal axis by a multiplying gear, or (as is now more usual) by means of a winch handle simply. The cushion is affixed to one horizontal metal cylinder, and the collector to another. It is necessary to insulate the axis in this machine, owing to its proximity to the ends of the conductors. Positive or negative electricity can be obtained with equal readiness by insulating either of the conductors, and connecting the other with the earth. Those who desire more minute information concerning the functions of the different organs of the frictional machine, are referred to Mascart, tom. ii. § 834, &c. In the same place will be found a description of the famous machine with double plates constructed by Cuthbertson for Van Marum, and still to be seen in Teyler's Museum at Haarlem. A description of another of Van Marum's machines will be found in the article "Electricity" in the Encyclopædia Metropolitana. We take this opportunity of calling the scientific reader's attention to that article, which contains a great quantity of very valuable matter. Much of the work of the earlier electricians that we have been obliged to pass over in silence is fully described there.

Friction

Electric machines have also been constructed of less costly materials than glass or even vulcanite-of cloth and paper, for instance -for an account of these, see Riess, Bd. ii. §§ 936, 937.

kept cool by a stream of water. The steam issues from these nozzles and impinges on a conductor armed with points for collecting the electricity. The boiler gets electrified to a high potential, and a torrent of dense sparks may be drawn from it. The machine far surpassed any ordinary electrical machine in the quantity of electricity furnished in a given time. By means of it water was decomposed, and the gases collected separately. It was reserved for Faraday to trace the exact source of the electromotive force. He demonstrated, by a series of ingenious experiments, that the electrical action arose from the friction of the particles of water in the condensed steam against the wood of the nozzles.1

Many experiments have been made on the electrification of sifted powders. We have already, in describing Lichtenberg's figures, alluded to some cases of this kind. As a rule, either the results ders, are very uncertain, or the conditions of the experiment very complicated, so that the experiments are, in most cases, more curious than valuable, from a scientific point of view. Such as desire it will find abundant indications of the sources of information in Riess, Bd. § 938 sqq., and Ency. Metrop., art. "Electricity," $193 sqq. One case of this kind, however, was so famous in its dey, that we ought to mention it. In the year 1840 a workman at Newcastle, having accidentally put one hand in the steam which was blowing off at the safety valve of a high-pressure engine boiler, while his other hand was on the lever of the valve, experienced a powerful electric shock in his arms. Armstrong investigated the matter, and was led to construct his famous hydroelectric machine. This apparatus consists simply of an insulated boiler for generating high-pressure steam, fitted with a series of nozzles,

results.

Remaining Cases. Of these the most important are atmospheric Miscel electricity, which belongs properly to meteorology, animal elec. laneous tricity, comprehending the study of the properties of the electrical fishes, and the electric phenomena of nerve and muscle. We have already indicated the literature of the former subject, and the latter belongs, for the present at least, to physiology. Evaporation, com. bustion, and in fact chemical action generally, have been brought forward by some experimenters as sources of electromotive force. About the last of all there is, of course, in one well-known case no doubt. As to the experiments generally alluded to under the other two heads-in particular, those of Laplace and Lavoisier, Volta, Pouillet, and others-there has been considerable difference of opinion, and we need not occupy space here with fruitless dis cussion of the matter. Similar remarks apply to the electrification caused by pressure, cleavage, and rupture.

Machines founded on Induction and Convection.-The oldest electric machine on this principle is the electrophorus of Volta, Electro 1775. This consists of a plate of resinous matter (now usually phorus. vulcanite) backed by a plate of metal, and a loose metal plate, which we may call the collector, fitted with an insulating handle. The vulcanite is electrified by flapping it with a cat-skin, the collector is placed upon it, uninsulated for a moment by touching it with the finger, and then lifted by the insulating handle. The collector plate is then found to be charged (positively) to a high potential, and sparks of some length may be drawn from it. The explanation of the action of the electrophorus is simple enough, if we keep clearly in view the experimental fact that the surface elec trification of a non-conductor, like vulcanite, will not pass to a metal plate in contact with it under ordinary circumstances. If the surface density of the electrification be very great, discharge to the metal may no doubt take place; and if the collector be kept for a very long time in contact with the vulcanite, it is said that it may become negatively electrified. In the normal state, however, the negative electricity of the vulcanite remains upon it, and the thin layer of air intervening between it and the collector forms the dielectric in a condenser of very great capacity, so that a quantity of electricity collects on the lower surface of the condenser very nearly equal to that on the vulcanite. The difference of potential between the plates is very small (just as in Volta's condensing electroscope, see above, p. 34). When the collector is raised it carries away the positive charge-the potential of which, owing to the decrease in the capacity of the collector, rises enormously. It is to be noticed that the potential of the charge on the vulcanite rises to a corresponding extent. This remark partly explains the remarkable fact that, when the collector is kept on the excited vulcanite, its electrification may be kept for a long time (for weeks under favourable circumstances), whereas it speedily dissipates if the vulcanite be left uncovered. According to Riess, the fact that a plate of metal laid on an excited piece of glass tends to preserve its electrification was discovered by Wilcke in 1762.

If each time we charged the collector it were discharged by contact with the interior surface of a hollow conductor A, it is obvious that we could raise A by a sufficient number of such contacts to as high a potential as we please, provided it were sufficiently well insulated. This remark brings Volta's electrophorus into the present category of electrical machines.

In the rest of the induction machines to be described the excited dielectric is dispensed with, and an electrified conductor substituted in its place.

The earliest apparatus that involved the principle of such machines Bennet's appears to have been Bennet's doubler. The principle of this doubler. apparatus may be explained thus. Let A and C be two fixed discs, and B a disc which can be brought at will within a very short dis tance of either A or C. Let us suppose all the plates to be equal, and Exp. Res., ser. xviii. 2075.

1

2 See Riess, § 1028 sqq., and Thomson's papers in Reprint already alluded to; also Ency. Metrop., art. "Electricity," § 219, for biblio graphy of older investigators.

See Riess, §§ 943 sqq.

This highly-descriptive title is Sir William Thomson's.

5 In most modern specimens this is rendered unnecessary by a brasa pin, which is in metallic connection with the metal backing of tho vulcanite, and comes up flush with the surface of the vulcanite, so as to touch the collector when it is in situ.

Phil. Trans., 1787