It will be noticed that, in the case of acids which on electrolysis evolve hydrogen at the kathode and, owing to secondary reactions, oxygen at the anode, decomposition starts with a potential difference of 1.7 volts. Experiments on the influence of the concentration of the solutions on the minimum E.M.F. required to produce continuous decomposition have shown that, in the case of solutions of acids where the value is about 1.7, this value is practically independent of the concentration. In the case of such acids as hydrochloric acid, however, where the value is considerably below 1.7, the minimum E.M.F. increases as the dilution is increased, and approaches the value 1.7 for very great dilutions. It is of interest to note that in a very dilute solution of HCl the gas liberated at the anode is no longer chlorine, but that a secondary reaction takes place and oxygen is evolved; so that at these great dilutions the electrolysis of HCl, as of the other acids, involves the evolution of H and O, and under these circumstances the minimum E. M.F. is the same for all.

CHAPTER XVI

CONTACT E.M.F. AND THE VOLTAIC CELL

If

N

A

C

545. Contact Electrification. — If a metal needle, a (Fig. 519), having the shape of half a quadrant electrometer needle, is suspended by a fine wire so as to be able to turn about a vertical axis through B, just above two metal semicircles, one of which, Z, is of zinc, and the other, C, of copper, then on electrifying the needle, if it is symmetrically arranged, no deflection will occur if the zinc and copper are insulated the one from the other. If, however, the zinc and copper are put in contact, either directly or through a conducting wire, the needle will be deflected. the needle is charged with positive electricity, the deflection will be away from the zinc and towards the copper, thus indicating that the zinc is at a higher potential than the copper. This difference of potential between the zinc and copper, as indicated by the charged needle suspended over the metals, is said to be due to contact electrification. The magnitude of the contact difference of potential does not depend on the time the metals are in contact, nor on the area of the surface of contact; it does however depend on the nature of the metals, both chemical and physical, and on the temperature. The nature of the surfaces of the metals which are exposed to the air also has an important bearing on the magnitude of the contact difference of potential. A list of the metals can

B

FIG. 519.

be drawn up such that any metal in the list when put in contact with any of the following metals is at the higher potential, but is at the lower potential when put in contact with any of the metals before it in the list. The following is such a list: Zinc, lead, tin, iron, copper, silver, gold. This list, which was first given by Volta, who discovered the contact effect, is called Volta's series.

If three metals, A, B, and C, are put into contact in pairs, the difference in potential between any two is equal to the algebraic sum of the difference in potential produced by the contact of each of the metals with the third. Thus suppose the difference in potential produced by the contact of A and B is 1, while that between B and C is p, then the difference of potential produced by the contact of A and C is p1+2.

This law can be very clearly exhibited by means of diagrams in which the potential of a metal is represented by the height of a rect angle. Thus in the case of the three metals, tin, copper, and iron, the difference in potential between tin and copper is 0.5 volts, the tin being at the higher potential; hence, if we take 1 cm. to represent a volt, we draw the rectangles (a), Fig. 520, such that the height of the tin rectangle is 0.5 cm. greater than that of the copper rectangle. The difference in potential between tin and iron is 0.3 volts, so that, the tin rectangle being drawn the same height as before, the iron rectangle will be 0.3 cm. lower, as shown at (6). The difference in potential between the copper and the iron will be two-tenths of a volt, and if the rectangle for the copper is drawn of the same height as in (a), the rectangle representing the iron will be 0.2 cm. higher, that is, it will be of the same height as in (b).

If we imagine the copper and the iron both put into contact with the same piece of tin, then it is at once evident, from a consideration of Fig. 520 (d), that the difference in potential between the copper and the iron is the same as it is when they are put in direct contact. Thus

the difference in the potential of any two metals is the same, whether they are put in direct contact or whether they are joined by means of a wire composed of another metal.

It follows from the above law that if we arrange a circuit of which the parts are of different metals, but the first and last metals are the same, then there will be no difference in potential between these end portions.

If, however, the first and last metals are different, say A and B, the difference in potential between these metals being p, then the dif ference in potential between the end metals will be equal top, although they are connected together by other metals.

It might at first sight appear, since we have two metals A and B at a difference of potential p, and that owing to the contact difference of potential they are kept at this constant difference, that on connecting A and B by means of a wire, a current would be set up in this wire. This, however, is not the case, for suppose we attempt to connect A and B by a wire of the metal A, then the difference of potential between the end of this wire and the metal B is p, but when the wire touches B,

owing to the contact, a difference in potential of p will be developed at the point of contact, and this difference of potential will prevent the difference of potential which exists between the metals A and B, forming the end of the chain, forcing electricity through the wire. The same can be shown to be true whatever the nature of the wire by which A and B are joined, so that by no arrangement of metals, all at the same temperature, can we obtain a current in a circuit which is composed exclusively of metals.

The case when we are dealing with the contact differences of potential between liquids, or between metals and liquids, is however quite different. Thus when copper is in contact with a solution of sulphuric acid, the copper is at the higher potential, while zinc, which is at the higher potential when in contact with copper, ought, if the liquid behaved as a metal would, according to the above law, to be at a higher potential than the sulphuric acid solution, instead of which it is at a lower potential. Hence it is possible to arrange a circuit composed partly of solid and partly of liquid conductors, such that a difference of potential exists between two parts of the circuit, even when these parts are connected by a conducting wire. Thus suppose we have a circuit composed of a plate of copper dipping in a solution of sulphuric acid, a plate of zinc also dipping in this solution, and a copper wire touching the zinc. The diagram of the potentials is shown in Fig. 521. The copper, Cu, is at a higher potential than the solution, H2SO4, while the solution is at a higher potential than the zinc, Zn. The zinc is at a higher potential than the copper wire, Cu', so that the wire is at a lower potential than the copper plate. Hence by this arrangement we have got two portions of the same metal (copper), which, owing to contact differences of potential, are at different potentials, and, since when the copper wire is put in contact with the copper plate we are dealing with the contact of the same metal, and therefore no contact difference of potential is produced which would annul the tendency of the existing difference of potential to cause electricity to move in the circuit, we have here an arrangement suitable for producing an electric current.

Cu H2SO4

Zn Cu

FIG. 521.

It is not even necessary that the contact of dissimilar metals occurs in the circuit, or even that two metals be employed, for a galvanic element can be produced in which no such contact of dissimilar metals occurs, or in which only a single metal is employed. Thus, when immersed in dilute sulphuric acid, copper is at a higher potential than lead, while when immersed in a solution of sodium sulphide (Na,S), copper is at a lower potential than the lead. Hence if we have two glass vessels, one containing dilute acid and the other a solution of sodium sulphide, and place a strip of lead so that one end dips in the acid and the other end

dips in the solution of the sulphide, while one plate of copper is placed in the acid and another in the sulphide solution, then, as shown by

Cu H2SO4 Pb

FIG. 522.

the diagram in Fig. 522, the copper is Cu at a higher potential than the lead in the acid in the vessel A, while the copper in the vessel, B, containing the sodium sulphide solution is at a lower potential than the lead. Hence the copper plate C is at a higher potential than the copper plate D, and if they are joined by a copper wire, a current of electricity will flow through the wire, although there is no contact of dissimilar metals.

Cu

546. Magnitude of the Contact Difference of Potential.-In a voltaic cell consisting, say, of a plate of zinc and a plate of copper in dilute sulphuric acid solution, there are three different contacts between dissimilar materials, namely zinc/copper, copper/acid, and acid/zinc, and hence we have to deal with three contact differences of potential. The question as to the relative magnitude of these three contact differences is one which has occasioned an immense amount of discussion. The question does not lend itself to experimental decision, for no method has as yet been devised, which is free from all objection, for measuring the contact difference of potential between two bodies without the intervention of one or more other media, although, as we shall see in § 549, this can be got over if we accept the ionic theory. Thus, in the experiment of the charged electrometer-needle suspended over the zinc and copper quadrants described in § 545, what is measured is not the potential difference between the zinc and the copper but the difference in potential between the air in the neighbourhood of the zinc and that of the air in the neighbourhood of the copper. Hence if we indicate the true contact difference of potential between zinc and copper by Zn/Cu and so on, then what is actually measured is the sum of the three contact differences of potential, air/Zn+Zn/Cu+Cu/air. Thus if the two quantities air/Zn and Cu/air are not both zero or equal and opposite, it does not follow that the quantity Zn/Cu is not zero, or at any rate very small, so that the two metal quadrants may be really at the same potential, and we need not necessarily have two parts of a conductor at different potentials when the electricity is at rest.

Although it is impossible to obtain an experiment showing that the difference of potential of about 0.7 volts, which is observed between the air in the neighbourhood of a piece of zinc and a piece of copper which are in contact, is really due to the fact that the metals themselves are at