5 -'8 



NATURE 



[September 28, 1891 



much in advance of English ones in this matter. Mr. Jones 

 gave an account of the system of teaching experimental physics 

 as now carried out under Mr. Rintoul at Clifton School. Re- 

 ferring to the subsidising of science teaching in secondary 

 schools, he pointed out that supervision of some kind must 

 accompany public aid. If the evil effects of examinations were 

 to be avoided, an efficient system of public inspection must be 

 developed, and the inspectors should be men with experience of 

 teaching work. Prof. Fitzgerald agreed with this ; and Dr. 

 Lodge expressed his regret that head-masters of schools could 

 not be compelled to attend and listen to discussions such as 

 this. The President, in concluding the discussion, said that 

 an elTort sliould be made to replace examinations by an intelli- 

 gent system of inspection. 



The last sitting was held on Tuesday the 19th. The Presi- 

 dent, as Secretary, submitted the report of the Electrical 

 Standards Committee. This report defines the unit ohm as 

 being the resistance of a column of mercury 106 '3 cm. long, and 

 of I4'452i grammes mass at 0° C. The B. A. unit is equal to 

 0'9866 of the ohm thus defined. The French authorities had 

 forwarded through M. Mascart four names which were proposed 

 for the ohm of the 106 '3 cm., and of these names the com- 

 mittee on the whole preferred "international." The resolutions 

 respecting the electrical units passed at the Edinburgh meet- 

 ing have now been accepted in Germany, France, Austria, Italy, 

 and ihe United States, and throughout the British empire. 

 Prof. Carey Foster said it should be known that the work of 

 the Committee was really the work of the President (Mr. 

 Glazebrook). 



On Slandixrds of Low EUctrkal Resistance. — Principal 

 J. Viriamu Jones described the method of determining the 

 ohm devised by Lorenz and used by Lord Rayleigh and subse- 

 quently by himself. The method consists in rapidly rotating a 

 copper disc coaxially in the mean plane of a standard coil, the 

 same cuirent being led through the coil and through the low 

 resistance which is to be measured. By varying the speed of 

 rotation the difference of potential between the centre and the 

 circumference of the disc can be made to counterbalance the 

 difference of ])otential at the ends of the resistance. Prof. 

 Jones pointed out that it is not only the most accurate method of 

 measuring the ohm, but that it is especially suited for the 

 measurement and production of very low resistances. Errors 

 are necessarily produced in stepping down from a standard 

 ohm, say by the potentiometer method, to a re- 

 sistance of I 1000 or 1/10,000 of an ohm. The Lorenz method 

 is sufficiently simple and accurate to be adapted for 

 the direct production of low resistances from a rotat- 

 ing disc and standard coil without going through the circuitous 

 process of stepping up to a standard ohm and down again. The 

 difficulty of making a good contact with the edge of the disc is 

 avoided by u-ing a tube with mercury running through it at 

 constant pressure. Difficulties were encountered in using the 

 electrically driven tuning-fork ; for although it vibrated 

 uniformly when once started, it was liable to a small change when 

 stopped and started again. A series of experiments on a re- 

 sistance of 1/20DO of an ohm was given in which the variation 

 from the mean of the extreme values was only i in 12, coo. 

 Lord Rayleigh expressed his pleasure at the extraordinary ac- 

 curacy now obtained by the method. In his own experiments 

 the electrically driven tuning fork, instead of being stopped and 

 started a^ain, was kept on all day, and compared at the begin- 

 ning with a free fork of the s.ame frequency (128). In a recent 

 paper Dorn has criticised the various methods used in the deter- 

 mination of the ohm, and has raised against Lorenz's method 

 the objection that particles of iron in the disc might affect the 

 result by altering the permeability inside the coil. This assumes 

 that the presence of such particles would introduce a direct 

 factor into the result, which would only be true if the whole 

 space inside the coil were so filled. 



Apparatus for Compnring nearly Equal Resistances. — Mr. 

 F. II. Nalder exhibited a modified Wheatstone Bridge 

 for comparing nearly equal resistances. In applying the 

 Carey Foster method only a small portion of the usual metre 

 bridge is brought into actual use, and in Mr. Nalder's 

 modification only the part thus used (about i decim. long) 

 is provided. This can be replaced by other wires of the same 

 length but of different diameters and therefore resistances ; of 

 course the resistance per unit length in each of these is known. 

 The comparison coils are wound in a single bobbin, so as to 

 avoid temperature errors ; these, and errors due to the thermo- 



electric effects, are materially reduced by the compactness of the 

 whole apparatus. Dr. O. Lodge described a new form of 

 galvanometer for physiological purposes. It was designed by 

 himself and made by Messrs. Nalder Brothers. The nerve cur- 

 rents excited by stimuli are exceedingly feeble and, even with the 

 so-called non-polarisable electrodes, the currents under investi- 

 gation are frequently masked by other effects. Physiologists 

 require an exceedingly sensitive ballistic galvanometer ; but they 

 appear generally to use needles which are far too heavy, and 

 galvanometers which are too highly damped, and which mani- 

 festly cannot be so delicate as undamped galvanometers. The 

 best form of galvanometer for their requirements is one which 

 contains a very light needle built up ol short pieces of highly 

 magnetised steel wire and in which the coils are small and are 

 wound up as close as possible to the needles. The instrument 

 exhibited had four such coils and four needles, forming an 

 astatic system suspended by a quartz fibre in a very weak field. 

 Compared with the usual galvanometers of the same resistance 

 its sensitiveness was at least twice as great. Prof. Boys has 

 shown that excellent definition can be obtained from a small 

 scrap of reflecting mirror ; and Lord Rayleigh has shown that a 

 pointer read by a microscope admits ol just the same degree of 

 delicacy as the mirror method. As biologists are accustomed to 

 looking through microscopes, Dr. Lodge suggested that they 

 might prefer to observe through a micrometer eyepiece a needle 

 with a bee-sting as pointer. Prof. Fitzgerald suggested that the 

 damping might be further reduced by hanging up the netdle in 

 a vacuous tube ; and that the polarisations might be swejit oat 

 by introducing capacities as in cable work. 



A Simple Interference Experiment. — There is a well-known 

 interference arrangement in which the object-glass ofa telescope 

 is covered by an opaque diaphragm containing two narrow slits. 

 An observer looking through the telescope at a radiant pointer 

 slit parallel to the two narrow slits sees a bright central band of 

 white light bordered by interference-bands. Lord Rayleigh 

 had investigated the part played by the telescope in this ar- 

 rangement, and found that the interference-bands can be equally 

 well obtained by using a plain brass or cardboard tube, having 

 at one end a single slit and at the other a double slit consisting 

 of two fine scratches on a piece of chemically silvered glass about 

 i/iooth of an inch apart. The President thanked Lord Rayleigh 

 for introducing such a simple form of interference experiment, 

 and said it should be more generally recognised that, inaimuch 

 as the eye contains a lens and screen, we can frequently do 

 without an observing telescope in optical experiments. 



On Specula for Reflecting Telescopes. — Dr. A. Shafarik com- 

 municated the results of investigations which he has carried on 

 since 1S70 with the object of producing specula having greater 

 tenacity than that of the Ross telescope. Silveredglass 

 mirrors produced by the Foucault method suffer rapid deterio- 

 ration in the air of large towns. The addition of phosphorus 

 is found to make bronzes harder and closer ; and the addition 

 of iron, nickel, or cobalt gives them a surprising degree of 

 tenacity. In general the strongest alloys are those which con- 

 tain the metals in atomistic proportions ; and even a small 

 deviation from this proportion appears to diminish the strength 

 considerably. The process of grinding specula differs from that 

 of grinding glass, for alloys are never homogeneous, they 

 are full of crystals, as can be shown by partially dissolving out 

 with acids. The relative tenacity of the Rosse speculum and of 

 two other alloys is given below : — 



R = CujSnj 

 ZN = CujSniNi 

 D = CujSnj + 4 per cent Zn. 



Strength = I 'cxj 



.. = 6-33 

 „ = 0-52 



Several members pointed out that what was really required was 

 a knowledge of the values of Young's modulus for the various 

 alloys investigated, and that it was doubtful whether this was 

 what the author referred to as " strength," or in what way the 

 measurements had been carried out. 



Prof. O. Lodge communicated a supplementary note on the 

 ether. He had been asked how could dust polarise light if 

 there was no mechanical connection between ether and matter? 

 But on the electro-magnetic theory there was no difficulty, for 

 light is not an ethereal oscillation but an electrical oscillation, 

 and if the dust has different values of /u and k from the ether it 

 may affect the wave. Mr. Trouton stated that dust particles 

 act as reflecting resonators with free periods. The President 

 had to confess that he did not fully understand the sense in 

 \ which Prof. Lodge used the word "mechanical," but con- 



NO. 1248, VOL. 48] 



