452 



NATURE l^ 



[Sept. 21, 1876 



The hospitality displayed at some of the excursions was 

 mag-nificent, and the foreign visitors had been most 

 cordially received. Prof Stokes, of Cambridge, proposed 

 thanks to the University of Glasgow for the very great 

 accommodation it had afforded to the Association ; the 

 motion was seconded by Dr. Carpenter. Sir William 

 Thomson proposed the vote of thanks to the President. 

 He thought Dr. Andrews's presidency would be be- 

 neficial to the Association in many ways. In his 

 address there were many things the serious and 

 permanent consideration of which would prove most 

 beneficial to the progress of science and of higher educa- 

 tion in the country. Dr. Allen Thomson, the President- 

 Designate for 1877, seconded the motion. Dr. Andrews, 

 in responding, expressed his gratification at the scientific 

 character of the meeting, which, he thought, would bear 

 comparison with any other. All the sections had been 

 above the average, and in Section A. numerous papers of 

 no ordinary importance were read. He referred especially 

 to a paper by Dr. Ker. of Glasgow, who had followed up 

 one of the most difficult researches of Faraday, and had 

 presented a paper of great originality and extreme value. 

 There had been little that was sensational in their pro- 

 ceedings, but he believed even the public at large would 

 greatly prefer true scientific work to excitement. 



This meeting has been notable for the attendance of 

 eight ex-presidents, viz., Prof. Stokes, Dr. Carpenter, Sir 

 William Thomson, Prof. A. W. Williamson, Sir John 

 Hawkshaw, Dr. JFjnnljrr Pr Tilnyd) nnd the Piikp nf 

 Ari_. "" 



REPORTS, 



Report of the Committee for Testing Experimentally the Exact- 

 ness »f OhrrHs Law, drawn up by Prof. Clerk Maxwell. — The 

 statement of Ohm's law is, that for a conductor in a given state 

 the electromotive force is proportional to the current produced. 



If we divide the numerical value of the electromotive force by 

 the numerical value of the current, the quotient is defined as the 

 resistance of the conductor, and Ohm's law asserts that the re- 

 sistance, as thus defined, does not vary with the strength of the 

 current. The difficulty of testing this law arises from the fact 

 that the current generates heats and alters the temperature of the 

 conductor, so that it is extremely difficult to ensure that the con- 

 ductor is at the same temperature when currents of different 

 strength are passed through it. 



Since the resistance of a conductor is the same in whichever 

 direction the current passes through it, the resistance, if it is not 

 constant, must depend upon even powers of the intensity of the 

 current through each element of the conductor. Hence, if we 

 can cause a current to pass in succession through two conductors 

 of different sections, the deviations from Ohm's law will be 

 greater in the conductor of smaller section, and if the resistances 

 of the conductors are equal for small currents they will be no 

 longer equal for large currents. 



The first method which occurred to the Committee was to 

 prepare a set of five resistance-coils, of such a kind that their 

 resistance could be very accurately measured. Mr. Hockin, who 

 has had great experience in measuring resistances, suggested 

 30 ohms as a convenient magnitude of the resistance to be 

 measured. The five coils, and two others to complete the bridge, 

 were therefore constructed, each of 30 ohms, by Messrs. Warden, 

 Muirhead, and Clark, and it was found that a difference of one 

 in four millions in the ratio of the resistance of two such coils 

 could be detected. 



According to Ohm's law, the resistance of a system consisting 

 of four equal resistances joined in two series of two, should be 

 equal to that of any one of the coils. The current in the single 

 coil is, however, of double the intensity of the current in any one 

 of the four coils. Hence, if Ohm's law is not true, and if the five 

 coils when compared in pairs with the same current are found to 

 have equal resistances, the resistance of the four coils combined 

 would no longer be equal to that of a single coil. 



A system of mercury cups was arranged so that when the 

 system of five coils was placed with its electrodes in the cups, 

 any one of the coils might be compared with the other four 

 combined two and two. 



After this comparison had been made, the system of five coils 

 was moved forward a fifth of a revolution, so as to compare the 

 second coil with a combination of the other four, and so on. 



The experiments were conducted in the Cavendish Laboratory 

 by Mr. G. Chrystal, B.A., Fellow of Corpus Christi College, 

 who has prepared a report on the experiments and their results. 



A very small apparent deviation from Ohm's law was observed, 

 but as this result was not confirmed by the mu;h more searching 

 method of experiment afterwards adopted, it must be regarded 

 as the result of some irregularity in the conducting power of the 

 connections. 



The defect of this method of experiment is that it is impossible 

 to pass a current of great intensity through a conductor without 

 heating it so rapidly, that there is no time to make an observation 

 before its resistance has been considerably increased by the rise 

 of temperature. 



A second method was therefore adopted in which the resist- 

 ance was compared by means of strong and weak currents, which 

 were passed alternately through the wires many times in a second. 

 The resistances to be compared were those of a very fine and 

 short wire inclosed ia a glass tube, and a long thick wire of 

 nearly the same resistance. When the same current was passed 

 through both wires, its intensity was many times greater in the 

 thin wire than in the thick wire, so that the deviation, if any, 

 from Ohm's law, would be much greater in the thin wire than in 

 the thick one. 



Hence if these two wires are combined with two equal large 

 resistances in Wheatstone's bridge, the condition of equilibrium 

 for the galvanometer will hz different for weak currents and for 

 strong ones. But since a strong current heats the fine wire much 

 more than the thick wire, the law of Ohm could not be tested 

 by any ordinary observation, first with a weak current and then 

 with a strong one, for before the galvanometer could give an 

 indication, the thin wire would be heated to an unknown extent. 

 In the experiment, therefore, the weak and strong current were 

 made to alternate thirty and sometimes sixty times in a second, 

 so that the temperature of the wire could not sensibly alter during 

 the interval between one current and the next. 



If the galvanometer was observed to be in equilibrium, then if 

 Ohm's law is true, this must be because no current passes through 

 the galvanometer, derived either from the strong current or the 

 weak one. But if Ohm's law is not true, the apparent equili- 

 brium of the galvanometer needle must arise from a succession 

 of alternate currents through its coil, these being in one direction 

 when the strong current is flowing, and in the opposite direction 

 when the weak current is flowing. To ascertain whether this is 

 the case we have only to reverse the direction of the weak cur- 

 rent. This will cause the derived currents through the galvano- 

 meter coil to flow both in the same direction, and the galvanometer 

 will be deflected if Ohm's law is not true. 



Mr. Chrystal has drawn up a report of this second experiment,, 

 giiring an account of the mode in which the various difficulties 

 were surmounted. Currents were employed which were sometimes 

 so powerful as to heat the fine wire to redness, but though the 

 difficulty of obtaining a steady action of the apparatus was much 

 greater with these intense currents, no evidence of a deviation 

 from Ohm's law was obtained, for in every experiment in which 

 the action was steady, the reversal of the weaker current gave no 

 result. The methods of estimating the absolute value of the 

 currents are described in the report. 



A third form of experiment, in which an induction coil was 

 employed, is also described, but though this experiment led to 

 some very interesting results, the second experiment gives the 

 most searching test of the accuracy of Ohm's law. 



Mr. Chrystal has put his result in the following form : — If a 

 conductor of iron, platinum, or German silver of one square 

 centimetre in section has a resistance of one ohm for infinitely 

 small currents, its resistance when acted on by an electromotive 

 force of one volt (provided its temperature is kept the same) is 

 not altered by so much as the millionth of a millionth part. 



It is seldom, if ever, that so searching a test has been applied 

 to a law which was originally established by experiment, and 

 which must still be considered a purely empirical law, as it has 

 not hitherto been deduced from the fundamental principles of 

 dynamics. But the mode in which it has borne this test not 

 only warrants our entire reliance on its accuracy within the limits 

 of ordinary experimental work, but encourages us to believe that 

 the simplicity of an empirical law may sometimes be an argument 

 for its exactness, even when we are not able to show that the law 

 is a consequence of elementary dynamical p rinciples. / ^ 



•^ ^h^f fprl— e f- ihi TH)dfth ~ tieporf~of the Committee Jor Ex' 

 ploring Kenfs Cavern, Devonshire. Read at Glasgow, Sep- 

 tember 8. — The Eleventh Report, presented by the Committee 



