70 



NATURE 



[November i6, 1899 



SOCIETIES AND ACADEMIES. 



London. 

 Physical Society, November lo.— Ordinary meeting held 

 in the Physical Laboratory of the Central Technical College 

 (by invitation of Prof. Ayrton). — Prof. Lodge, F.R.S., President, 

 in the chair. — Mr. F. S. Spiers read a paper on contact elec- 

 tricity. The object of the paper was to determine, in a more 

 satisfactory manner than has hitherto been attempted, the part 

 played by the medium in the potential difference which arises 

 when two dissimilar metals are put in contact. The first ex- 

 periments were made with a piece of apparatus used by Profs. 

 Ayrton and Perry about twenty years ago. This apparatus, in 

 which the metals in contact are capable of a rotation of 180" about 

 a vertical axis, and are placed between two vertical inductors con- 

 nected to a quadrant electrometer, was afterwards considerably 

 improved, and the compensation arrangement of Lord Kelvin 

 was introduced so as to measure the potential differences by a 

 hull method. The metals first used were platinum and zinc, 

 but on account of the low melting point of the latter metal it 

 was replaced by aluminium. In order to try and remove the 

 air sheets which cling to the surfaces of the metals, the tube 

 was repeatedly heated and exhausted. The potential difference 

 between the plates was found to gradually fall as this was done. 

 It was proved that this was due to the oxidation of the alu- 

 minium, for on cleaning its surface the original effect was again 

 obtained. Attempts were then made to remove the oxygen by 

 displacing it with hydrogen ; but after four washings with pure 

 dry gas and at low pressures there was still enough oxygen 

 left to completely oxidise the aluminium. The oxide of alu- 

 minium is not decomposed by hydrogen at a bright red heat. 

 It was therefore decided to substitute iron and burn out the 

 oxygen with hydrogen by encasing the lower part of the ap- 

 paratus in a copper tube, and heating to bright redness with 

 a blowpipe flame. By this means the value of the Volta 

 effect between iron and platinum in an atmosphere of 

 hydrogen was found to be o'b of a volt, the platinum 

 being positive to the iron. This result is different both in 

 magnitude and sign to that obtained when air is the medium. 

 The Chairman said he had given the subject of contact electricity 

 some attention during the last fifteen years, and the author had 

 performed a valuable series of experiments which he should have 

 liked to have seen done several years ago. He had always felt 

 that a vacuum would never get rid of the condensed air films. 

 The burning-out process used had provided the most trustworthy 

 results upon the subject. Dr. Lehfeldt pointed out that the 

 action of hydrogen upon ferric oxide was a limited one, and that 

 it was impossible to bring about complete deoxidisation in that 

 manner. At a dull red heat the ratio between the water vapour 

 and oxygen present is about 20 to i . Prof. Perry expressed his 

 interest in the experiments, but said that they had not affected his 

 opinion upon the nature of the Volta effect. Prof. Armstrong said 

 he was not wholly satisfied with the results, although a substantial 

 approach to a solution had been made. The author had fully 

 realised the difficulties of the experiments, but he had treated 

 the matter as a surface gas effect, and had not guarded against 

 moisture. Gases must be both dirty and moist before chemical 

 action can take place, and we cannot expect to arrive at a 

 solution of the problem until we have removed not only oxygen 

 but dirt and moisture. It is impossible to completely exhaust 

 the apparatus, and a number of molecules must always be left 

 which is more than necessary to produce the Volta effect. 

 Moisture can never be got rid of by exhaustion. The method 

 of Dewar of using liquid oxygen or liquid hydrogen would get 

 rid of gases and water vapour, and in this manner it would be 

 possible to perform experiments which could be regarded as 

 final. If the effect disappeared at low temperatures it might be 

 urged that the temperature was too low for it to be produced. 

 The author must have been dealing with combination effects, 

 for it had been proved that hydrogen alloyed both with 

 platinum and iron at a dull red heat. Mr. Cooper said he 

 would like to see the experiments repeated after pre- 

 cautions had been taken to remove nitrogen from the apparatus. 

 Prof. S. P. Thompson said he had recently taken part in a dis- 

 cussion upon the subject with some earnest followers of the old 

 contact theory. They uphold that the property of metals which 

 determines the potential difference when two are put in contact 

 is as fixed and definite as other physical properties, such as 

 density, and that the potential difference observed in air is 

 approximately the same as the true potential difference. 



NO. 1568, VOL. 61] 



It has been agreed to call the former the apparent poten- 

 tial difference. Prof. Thompson said that according to 

 Pellat the real Volta effect was near to the effect ob- 

 served in air. In circuits formed of metals there are other 

 electromotive forces of the order of a millionth of a volt. 

 The chemical electromotive forces in a circuit are of the order 

 of a volt. The value of the Volta effect derived from thermo- 

 dynamical considerations concerning the Peltier effect is much 

 smaller than observed chemical potential differences. If, how- 

 ever, we take into account not only the Peltier effect but also 

 the Thomson effect, we will have other terms entering into the 

 equations which may tend to give a value more nearly equal to 

 a volt. Prof. Thompson said that in observing chemical 

 E.M.F.s the Peltier effects did not come into the question 

 because of their smallness compared with the value of the 

 chemical effect. Prof. Perry pointed out that the Peltier effect 

 was not distinct from the Volta effect, but was simply the dif- 

 ferential coefficient of it. The Chairman said that if a circuit 

 containing Peltier effects were treated thermodynamically as if it 

 were a reversible heat engine, we could arrive at an equation 

 connecting the value of the Peltier effects with the rate of 

 change of the whole electromotive force in the circuit with 

 temperature. The electromotive force which changed was not 

 necessarily the Volta effect. Prof. Perry said he thought it 

 was. Prof. Ayrton suggested that an advance might be made 

 in the theoretical side of the question if the Chairman were 

 to put in writing his objections to the statement that theE.M.F. 

 concerned was the true Volta effect. The extent of the Peltier 

 effect proves the variation of the Volta effect with temperature ; 

 but because it is small it does not necessarily follow that the 

 Volta effect is small. Where the Volta effect is a maximum or a 

 minimum the Peltier effect vanishes. The experimental work 

 of the paper did not go far enough to convince him of the nature 

 of contact electricity. Before we can hope to prove anything 

 with respect to the two theories, we must be able to get a cyclic 

 change of events ; that is to say, we must be able to change our 

 surfaces and media in a perfectly definite manner so as to be 

 able at any time to return to the particular state from which we 

 started. Prof. Everett said that as th6 variation in the potential 

 difference between two metals in a medium was probably due to 

 slow chemical action which caused the metals to become less 

 and less susceptible, he should expect that changing backwards 

 and forwards from one medium to another would give to the 

 potential difference an oscillatory variation gradually becoming 

 smaller and smaller. The Chairman said he would like to see 

 experiments showing a cyclic effect similar to that mentioned by 

 Prof. Ayrton. The difficulty in these experiments is to avoid 

 chemical action. Chemical action is not necessary to get the 

 Volta effect. The effect would be greatest in dry gas. Moisture 

 tends to reduce the effect, and that is why its presence is un- 

 important. Prof. Callendar expressed his interest in the surface 

 character of the effect and its independence of the manner in 

 which the plates were touched. Dr. Stansfield suggested gold 

 as a suitable metal to be experimented on because of its non- 

 oxidisability. Mr. Spiers, in replying, referred to Dr. Lehfeldt's 

 assertion that the whole of the oxygen cannot be removed by 

 hydrogen. In his experiments, however, there was very little 

 ferric oxide and a large quantity of hydrogen, and although it 

 was possible that all the oxide was not reduced, still a large 

 portion of it was. The experiments were to be carried on, and 

 attempts would be made to get a cyclic effect. — A paper on 

 the heat of formation of alloys was postponed until the next 

 meeting. 



Manchester. 

 Literary and Philosophical Society, October 31.— Prof. 

 Horace Lamb, F.R.S. , President, in the chair. — Dr. Broad- 

 bent introduced the subject of the well at Giggleswick, known 

 for the ebb and flow of its water, and asked whether an explan- 

 ation of what is known locally as the " silver thread" could be 

 offered by any member. The well consists of a stone cistern, 

 at the top and back of which the water enters from the Giggles- 

 wick Scars, there being two small outlets about half-way down 

 each side of the tank and opposite each other. Under certain 

 conditions there appears extending through the water, from one 

 outlet to the other, a thread apparently formed of air. Prof 

 O. Reynolds suggested that the phenomenon might be explained 

 by the inflow producing a circulation of the water having its 

 vortex parallel between tne two outlets, the reduction of pressure 

 thus permitting a passage of air from one orifice to the other. — 

 Mr. J. Cosmo Melvill communicated a paper ^by Mr, Peter 



