December 31, 1891] 



NATURE 



213 



what form of instrument is capable of giving the most accurate 

 results. 



For practical purposes there can he no doubt that electrical 

 resistance thermometers, which are much easier to read and 

 manipulate, and which are, at the same time, exceedingly con- 

 stant over a very wide range, would be much more convenient 

 as standard instruments. But for theoretical work it is always 

 necessary to reduce their indications to the scale of absolute 

 temperature. 



With this object the writer has been for some years engaged 

 in endeavouring to construct an air-thermometer which should 

 be capable of reading to a degree of accuracy comparable with 

 that attained by the use of electrical-resistance thermometers. 

 This he believes that he has at length succeeded in securing by 

 the adoption of the modified and compensated form of differ- 

 ential air-thermometer described in the paper. 



The common and familiar form of differential air-thermometer 

 consists essentially of two equal bulbs, communicating with 

 opposite limbs of a U-tube of small bore containing sulphuric 

 acid, which serves to indicate the difference of pressure between 

 them. If the standard bulb be kept in melting ice, so that its 

 temperature is constant, it is possible, by using a kathetometer 

 microscope, to read small changes of temperature in the thermo- 

 metric bulb with an accuracy of the order of a thousandth of a 

 degree. 



In order to make the instrument capable of reading over a 

 wider range, it is only necessary to add an auxiliary bulb, as in 

 the ordinary " constant-pressure" type of air-thermometer, into 

 which the air from the thermometric bulb is allowed to dilate. 

 The auxiliary bulb is provided with taps, through which mercury 

 can be introduced or withdrawn in weighed quantities, to equalize 

 the pressures. The dilatation of the air at constant pressure 

 can be very accurately measured by weighing the mercury dis- 

 placed. This form of air-thermometer has the advantage of 

 being entirely independent of barometric readings. A great 

 deal of trouble is thus saved ; moreover, it is certain that a much 

 greater degree of accuracy can be attained in this way in the 

 measurement of a volume than in the measurement of a pressure 

 by means of a mercury manometer, as in the " constant-volume " 

 type of air-thermometer. 



With almost every form of air-thermometer, some part of the 

 air contained in the connecting tubes is necessarily exposed to 

 temperatures different from those of the bulbs. In accurate 

 work a correction must always be applied for this by calibrating 

 the connecting tubes and estimating their mean temperature. 

 This correction, however, is exceedingly troublesome to apply, 

 and becomes a very serious source of uncertainty in attempting 

 to work to a thousandth of a degree. 



It is, perhaps, the greatest advantage of this particular form 

 of differential air-thermometer, that this troublesome and un- 

 certain correction can be completely eliminated both from the 

 observations and from the calculations by simply duplicating the 

 connecting tubes — that is, by making the thermometric and 

 standard bulbs communicate with similar sets of connecting 

 tubes fixed side by side in such a way that their mean tempera- 

 tures are always equal. Provided that the two bulbs contain 

 equal masses of air, and that their pressures are adjusted to 

 equality, any change in the temperature of the connecting tubes 

 will affect both equally, and will not, therefore, alter the read- 

 ing of the pressure-gauge.^ In this way not only is the work of 

 taking and reducing the observations immensely simplified, but 

 the results are also rendered much more accurate. 



The form of instrument above described is designed for the 

 most accurate work. For rough purposes, and especially for 

 limited ranges of temperature, for which the auxiliary bulb can 

 be dispensed with, much simpler instruments may be constructed 

 and compensated on similar principles. 



In ordinary work it would be inconvenient to have to keep 

 the standard bulb always at a constant temperature. The 

 necessity for this may, however, be avoided by adjusting the 

 quantity of sulphuric acid in the pressure gauge so that its ex- 

 pansion compensates for the increase of pressure in the standard 

 bulb due to rise of temperature of the surrounding air. When 

 the instrument is thus compensated, one of the tubes of the 

 pressure gauge can be directly graduated in degrees of tempera- 

 ture. The indications are then as easy to read as those of a 

 mercury thermometer. Such thermometers are very convenient 

 for rough work at temperatures beyond the range of mercury 



' The equations and conditions of compensation ;are fully given in the i 

 paper. > 



thermometers. They can be made with a range from 300°- 

 500' C.^ (500° -900° F.), and will read to a tenth of a degree at 

 450" C. They are practically free from change of zero, and if 

 properly compensated their indications are very reliable. Since 

 the connecting tubes are compensated, they can be made of con- 

 siderable length, and even of flexible material, such as compo- 

 tubing, without much loss of accuracy. This is often a matter 

 of great convenience, especially in high temperature work. 



" Repulsion and Rotation produced by Alternating Electric 

 Currents." By G. T. Walker, B.A., B.Sc, Fellow of Trinity 

 College, Cambridge. Communicated by Prof. J. J. Thomson. 



The author described the following experiment : — A sheet of 

 copper is placed so as to half cover an alternating magnetic 

 pole. Upon this, near the pole, is laid a hollow sphere of 

 copper. The electro-magnetic action produces a couple so 

 powerful that the friction of rotation is overcome, and the 

 sphere spun round. 



In order to throw light on this, after a theorem in § 2 as to the 

 kind of currents set up in a conductor, I have considered a 

 number of cases. A thin circular infinite cylindrical shell lies 

 in an alternating field of currents parallel to its axis, and the 

 couple upon it is found. The result is applied to give the 

 couples on two such shells in the presence of a parallel current 

 and of a pair of currents forming an electro-magnet. 



The couple in action upon a thin spherical shell in a general 

 periodic field has next been found, and is applied to give the 

 couples on two thin shells under the influence of — 



(i.) An alternating current in a straight infinite wire. 



(ii.) A pair of such currents forming an electro-magnet. 



(iii.) An alternating magnetic pole. 



(iv.) An alternating electro-magnet of very short length. 



Chemical Society, November 19. — Sir Henry Roscoe, 

 F.R.S., in the chair. — The following paper was read : — Iron 

 carbonyl, by L. Mond, F.R.S., and Dr. Langer. An account 

 of this paper has already appeared in Nature of November 26 

 (p. 89). — A lecture was then delivered on colour photometry, 

 by Captain Abney, C.B., F.R.S. According to the lecturer, 

 the colour of a body, when viewed in a light of standard 

 quality, is known when {a) its luminosity, {b) its hue, and (<r) 

 its purity, or the extent to which it is freed from admixture with 

 white light, are known and expressed by numbers. The 

 luminosity of a colour can be given in absolute number by 

 referring it to the standard of white. The standard of white 

 employed is a surface coated with zinc oxide. It is also necessary 

 to employ a standard light in these experiments, and the light 

 recommended is that from the crater of the positive pole of the 

 electric light, or from a petroleum lamp, when the illumination 

 need not be so intense. The luminosity of the pure spectrum 

 colours may be measured by what the author calls the colour 

 patch apparatus, which is described in the Phil. Trans., 1886, 

 and in his book on " Colour Measurement and Mixture." The 

 luminosity of a colour is not the same when viewed from all 

 parts of the eye. The luminosity of any pigment on paper can 

 be found by rotating it with two or three colours, red, emerald- 

 green, and ultramarine. The colour of a pigment can be re- 

 ferred to the spectrum colours by measuring the absorption. 

 The mixture, in varying proportions, of red, green, and violet of 

 the spectrum, makes white. Any other colour can be matched 

 by the mixture of the same three colours. Since three colours 

 will make white, and the same three colours will make a match 

 with an impure colour, every colour in nature can evidently be 

 matched by mixing not more than two of these colours with a 

 certain proportion of white light, and if these colours be red and 

 green, or green and violet, the colour can be matched by one 

 spectrum colour and white light, since there is some intermediate 

 colour which has the same hue as the mixture of these two 

 colours. Hence any colour may be expressed in terms of white 

 light and one spectrum colour, the latter in wave-lengths and the 

 former in percentage of luminosity. The lecturer peTirformed 

 experiments in illustration of all the points brought forward. 

 The importance of using some uniform light was insisted upon 

 throughout. In conclusion the lecturer claims to have demon- 

 strated that the reference of colours to numbers is not only 

 possible but easy. 



December 3.~Prof. A. Crum Brown, F.R.S., in the chair. — 

 The following papers were read : — Phosphorous oxide, Part ii., 



' Kn air-pyrometer and also a long-distance thermometer of this pattern 

 are made by Mr. J. J. Hicks, of Hatton Garden, E.C. 



NO. II 57. VOL. 45] 



