August ii, 1910] 



NATURE 



195 



in the neighbouring counties, and the services of its staff 

 are placed at the disposal of farmers investigating new 

 conditions or special points arising out of their farming 

 operations. Under the regulations of the Scotch Education 

 Department, the college is recognised as a central institu- 

 tion to which students may be sent by burgh and county 

 education committees. Instruction in poultry-keeping has 

 been considerably developed. The interest in school 

 gardening grows, and seventy-three gardens have been laid 

 out with the assistance of the college staff. The experi- 

 ments and demonstrations carried on throughout the 

 neighbouring district are described in the calendar, and give 

 proof of the practical nature of the work undertaken. 



SOCIETIES AND ACADEMIES. 



London. 

 Physical Society, July 8.— Prof.. H. L. Callendar. 

 F.R.S., president, in the chair. — Prof. H. L. Callendar : 



The radio-balance : a thermoelectric balance for the abso- 

 lute measurement of radiation ; with applications to radium 

 and its emanation. In this apparatus, which was first 

 constructed in 1905, and was briefly described in an article 

 on radiation contributed to the " Encyclopaedia Britannica," 

 heat supplied by radiation is directly compensated by the 

 Peltier absorption of heat in a thermo-junction through 

 which a measured electric current is passed. In the 

 simplest form of the instrument, radiation admitted through 

 a measured aperture, 2 mm. diam., falls on a small copper 

 disc 3 mm. diam. by 0-5 mm. thick, to which two thermo- 

 junctions are attached, forming a Peltier cross. One couple 

 is connected to a sensitive galvanometer for indicating 

 changes of temperature. The other is connected to a 

 battery and rheostat in series with a millammeter or 

 potentiometer for measuring the current required to reduce 

 the deflection of the galvanometer to zero. In practice, 

 two similar discs with similar connections are mounted 

 side by side in a thick copper bo.\, and are balanced 

 against each other in order to avoid changes of zero due 

 to exposure to sunshine, or rapid variations of tempera- 

 ture. The advantages of the disc radio-balance are that it 

 is very simple to construct and easy to reproduce without 

 material variation in the reduction constants. It is very 

 suitable for measurements of solar radiation, or strong 

 sources, but is insufficiently sensitive for weak sources ; 

 and the absorption coefficient a must be determined by 

 comparison with a standard. In the cup radio-balance, the 

 radiation is received in a copper cup 3 mm. diam. by 

 10 mm. deep, so that the absorption coefficient is practically 

 equal to unity. Greater sensitiveness is secured by 

 employing a pile of several couples, insulated from the 

 cup. in place of the single balancing couple. External dis- 

 turbances are eliminated by employing a pair of cups, 

 similarly mounted but oppositely connected, enclosed in 

 a thick copper cylinder. The Joule effect, represented by 

 the C"R term in the equation, is automatically eliminated 

 by passing the same current in series through the opposing 

 Peltier Junctions soldered to the bottom of the cups. The 

 cup exposed to radiation is cooled, and the cup screened 

 from radiation is heated, by the Peltier effect, while both 

 are equally heated by the Joule effect. A complete 

 observation involves reversing the current and switching 

 over the radiation screen, in order to eliminate any differ- 

 ence of sensitiveness of the two piles. By observing the 

 neutral current, each cup can be used separately, as with 

 the disc balance, but the disc balance cannot be used 

 with the Peltier couples connected in opposition unless the 

 balancing couples are insulated from the discs. The cup 

 r.-idio-baIance is sensitive to less than a tenth of a micro- 

 watt, and is very suitable for measuring the heat evolved 

 by small quantities of radio-active substances. It was 

 applied to radium at Prof. Strutt's suggestion, and Prof. 

 Rutherford has kindly supplied samples of emanation, and 

 has determined the value of the radium sample employed 

 by comparison with his own standards. The second 

 sample of emanation had only just come to hand, and the 

 absolute values had not been finally reduced at the time 

 the paper was read ; but it appeared from the preliminary 

 reductions that the heat evolution of radium in terms of 

 Prof. Rutherford's standards was much greater than that 



NO. 2128, VOL. 84] 



given by previous observers. — Dr. A. Russell : The con- 

 vection of heat from a body cooled by a stream of fluid. 

 .Attention is directed to certain deductions made by 

 Boussinesq from the mathematical theory of the conduc- 

 tion of heat in liquids. Complete proofs are given of 

 Boussinesq 's formulae, stress being laid on their limitations, 

 and some of their practical applications are pointed out. 

 It is proved that when a hot body is immersed in a stream 

 of liquid flowing with constant velocity, the cooling is 

 proportional to the difference of temperature between the 

 body and the liquid. Newton proved experimentally in 

 1701 that this law was true for the case of a hot body 

 being cooled by a draught of air. He enunciated his law 

 with reference to the forced convection of heat from a 

 bodv and not, as is often stated, to the natural free con- 

 vection from it. Lorenz has shown that in special cases 

 the natural convection of heat will vary as the i-25th 

 power of the difference of temperature. Provided that the 

 velocity of the cooling draught is kept constant between 

 certain limits, Compan has shown that Newton s law is 

 very approximately true even when the difference of 

 ^emperamre is as high as 300" C. Another deduction from 



hT'^formulae proved" in the paper is that the cooling is 

 very approximately proportional to the square root of the 

 velocity of the convection current. The author gives the 

 solution of the problem of the heating of a liquid flowing 



?eadUy with a velocity less than the critical velocity 



hrou«h a cylindrical tube which is maintained at constant 

 temperature.' It is shown that, in -any prac ical cases 

 the heating power of the tube varies as R0^/s,rk\l, where 

 R is the rid'us of the tube, 8 the difference of temperature 

 between the tube and the liquid, 5 the specific heat, ^ the 

 d nifty, /c the conductivity, V the velocity o flow and 

 the length of the tube. It is proved that if a wire oe 



mmersed in a stream of liquid with its length at right 

 Ln'es o he direction of flow, the electric current which 

 w^fl fuse the wire varies as the i-25th power of the 

 diameter o the wire. Finally, the effect on the coohng 

 oTn electrically heated cylinder by a stream of liquid, of 

 ^utt^ng an insuring wrapping round it ^^ '^^"f^^^'^^Jl 

 is shown that in certain cases the effect of this procedure 

 to lowe the temperature of the cylinder an effec which 

 can be easily demonstrated experimentally. In order to 

 "mplifv the mathematical work, only the case o '"com- 

 nrTssble fluids is considered. Experimental results, how- 

 ^^er obtained by various physicists -e ^^uoted to show 

 that some of the formula; are approximately true tor tne 



00 ing of heated bodies by convection with currents of 

 '?°_Prof S P. Thompson : Hysteresis loops and Lissa- 

 Tous's figure , and on The energy wasted in a hysteres. 



000 Attempts have been made to find an explanat on of 



riorms o"'the looped curves which express the hys eres. 

 exhibited by iron and steel when subjected to cyc'es <Dt 

 magnetisation. Physical explanations ^o/ccount for their 

 c^eneral shape have been given by Ewing and Hopl^'"^°"' 

 ^nd M Pierre Weiss has put forward an electronic theory 

 io 'ac'^ount for the princip'^1 features. J^c -tho^^how 

 that any hysteresis loop can be analysed into a harmomc 



erts of closed curves corresponding '<> '^e .v-uni^^iS 

 in the analysis of the current wave, and *eir constituents 

 are examined in the paper. A number of e^'^'^P'f ° 

 hysteresis loops were chosen and subjected to analyses 

 The loops chosen related to various ^>nds of ^on ^^^ 

 steel, hard and soft, solid and •'^"l'"^'^^^; . ^f.^fsTmpIe 

 various methods. In carrying out the analysis the snnple 

 approximate method described by the author (Proc^Phys^ 

 Sor vol xiv ) was used. Details are given of the analysis 

 o'f^virlotrojs, the effect of eddy currents ot^ .e and 

 form of the loops is discussed, and an account is =.ve 



*u:ret^a1e%';:^'w. HN^cLT'-ThT^nT^^^^^^ 

 of certafn detectors used in wireless telegraphy^ The 

 paper is a record of the results of ^"'''^"ZXticlel^c^or, 

 tion into the physical properties of the electrolytic de^^ 

 the zincite rectifier, the carborundum rectifier ana 

 thermoelectric detector consisting o a light c°"tact bemeen 

 graphite and galena. The conditions of the experiments 

 have been generally identical with those arising n the 

 ordinary employment of the detectors, and, in particular 

 the quantities of energy given to the instruments, in the 



