December 19, 1907] 



NA TURE 



159 



EXHIBITION OF PHYSICAL APPARATUS. 



THE third annual exhibition of physical apparatus, held 

 under the auspices of the Physical Society of London 

 at the Royal College of Science on December 13, was an 

 unqualified success. Notwithstanding the inclemency of 

 the weather there was a good attendance, many members 

 of the society coming up from distant towns in order to 

 take advantage of the opportunity of inspecting the 

 apparatus itself in lieu of looking through the catalogues 

 of so many makers. Printed and verbal information was 

 available in abundance, and in connection with the former 

 it may not be out of place to offer a word of advice to 

 makers. .Any catalogue, however carefully compiled, is in 

 the case of a progressive maker out of date a few months 

 after publication, and is generally supplemented by separate 

 sheets sent out to customers. Very few makers have these 

 -lii-ris cut the same size as their catalogues, and fewer 

 ~iill provide clips at the ends of their catalogues by means 

 of which the additional sheets, sent out punched in the 

 left-hand margin, can be permanently incorporated in the 

 catalogue. They lie about on desk's instead until they 

 look dustv and disreputable, and are then consignee! to the 

 waste-paper basket, and the information contained in them 

 i> forgotten. 



.As one would naturally expect, the exhibition was 

 strongest on the electrical side, but other branches were 

 not neglected. In general physics, the silica ware ex- 

 hibited by Messrs. J. J. GriiHn and Sons attracted a con- 

 siderable amount of attention. Bowls of 6 inches 

 liameter, boiling flasks of 3 inches, and tubes of all kinds 

 in now be made of transparent silica, while much larger 

 objects are made of the opaque variety. 



Messrs. C. F. Casella and Co. exhibited a telemeter with 

 Ml 8-feet base, arranged to measure both distances and 

 differences of level, the telescopes rotating about the base 

 and the base about a vertical axis through its centre to 

 I liminate errors. Thev showed also a direct-reading 

 .inemometer on which the revolutions are given by an 

 .iidinary engine counter. Messrs. Elliott Bros, exhibited 

 iliiir new " motormeter, " or speed indicator, for motor- 

 [rs, which is driven from a flat rubber ring attached to 

 lie front off-side wheel of the car by means of a friction 

 A lieel and flexible shaft. 



In heat, the most interesting exhibit was that of the 

 Meteorological Office, which consisted of balloon and kite 

 meteorographs and traces obtained by means of them. 

 Mr. Dines's instrument for recording pressure and tempera- 

 iire on a square inch of thin copper weighs one ounce 

 • Illy, and is most ingenious. Records of ascents of 18 to 

 ji> kilometres, made at the same time at four stations in 

 this country, showed a fair agreement in the temperatures 

 at the same heights over the four stations at comparatively 

 low levels, but considerable differences at high levels. 



In photometry there appeared to be a general adoption 

 of the flicker photometer and of the inclined screen method 

 of varying the effect of one of the sources. The move- 

 ment of the screen is effected by means of a cam rotated 

 by a milled head outside the photometer box, to which a 

 pointer reading on a scale marked directly in candle- 

 power is attached. The uniformity of the divisions is 

 secured by the shape of the cam. 



Messrs. A. Hilger, Ltd., exhibited a large spectroscope 

 the telescope of which was moved by a tangent screw 

 graduated on the head directly in wave-lengths. They 

 also had on view a Fabry and Perot interferometer with 

 the interference bands visible, so that the displacement pro- 

 duced by separating the plates could be observed. 



Two new photographic lenses giving very flat fields 

 were exhibited, the " Isostigmar " by Messrs. R. and J. 

 Beck, and the " Homocentric " by Messrs. Ross, Ltd. 



The work on radio-activity, which is being carried on 

 so vigorously, has raised the electroscope to a position 

 undreamed of a dozen years ago, and amongst the many 

 new forms it now takes mav be mentioned one constructed 

 by Mr. C. W. Cook, of Manchester, for Prof. Rutherford, 

 and exhibited by Messrs. J. J. GriflSn. It contains a 

 romparfment below the leaves in which the radio-active 

 material to be investigated can be placed. 



Resistance bridges for the most accurate work appear to 

 be tending towards the enclosed type, with oil circulation 



NO. 1990, VOL. yyl 



to ensure uniformity of temperature. The Cambridge 

 Scientific Instrument Co. showed a Callendar and Griffiths 

 bridge in which plug contacts were replaced by mercury, 

 also enclosed, to prevent the mercury getting to the brass- 

 work. 



Several makers seem to be alive to the possibilities of 

 the flat form of resistance coil owing to its compactness 

 and freedom from inductance and capacity. Mr. L. 

 Miller's machine for winding the wire of induction coils 

 in flat vertical sections, the wire passing from outside to 

 inside and back again without a break throughout the 

 whole length of the coil, seems to make it possible to- 

 build larger coils without insulation troubles arising. His 

 mica-disc valve, which interposes a disc of mica in a 

 short air gap in the secondary circuit of the coil during, 

 the make, and so cuts down the make current that the 

 secondary current is practically unidirectional, should prove' 

 a great aid in vacuum-tube work. 



Moving magnet galvanometers show a tendency to take 

 the Broca form. In which astaticism is secured by making; 

 the poles between the coils consequent poles at the centres, 

 of two magnets placed vertically. Instruments so con- 

 structed were shown by the Cambridge Scientific Instru- 

 ment Co. and by Messrs. Clark Fisher and Wadsworth. 

 A very useful addition to the moving coll type of galvano- 

 meter was exhibited by Messrs. Gambrell Bros. It con- 

 sists of a resistance within the galvanometer case, which 

 when placed across the terminals of the Instrument renders 

 it aperiodic. One end of it is connected to one terminal 

 of the instrument, and the other to a third terminal, so 

 that it mav also be used to diminish the sensitiveness of 

 the galvanometer. 



Messrs. Paul exhibited a Campbell vibration galvano- 

 meter, w-hich is a inoving coil instrument of very short 

 period, the control being of the bifilar type, and the 

 amplitude of the oscillations being observed in working 

 with the instrument. Other Instruments for small alter- 

 nating currents were Duddell's thermo-ammeter, on the 

 same principle as his thermo-galvanometer, shown by the 

 Cambridge Scientific Instrument Co., and Cohen's 

 barretter, shown by Mr. R. W. Paul. This instrument is 

 of the bolometer type, the filaments the resistances of 

 which are changed by the current to be measured being 

 those of the 24-volt lamps used on telephone switchboards. 



Of instruments intended for commercial work, the iron- 

 clad indicating wattmeter shown by Messrs. Nalder and 

 Thompson may be mentioned, as it illustrates the present 

 tendency to secure larger torques by placing the moving 

 coil In the field of a laminated series or shunt electro- 

 magnet. 



Messrs. Nalder Bros, exhibited a compact testing set 

 weighing only 14 lb., capable of measuring insulations 

 up to 2000 megohms with 100 volts, and Messrs. Ever- 

 shed and Vignoles several of their " meggers " of various 

 ranges up to 1000 megohms. 



The Physical Society is to be congratulated on the 

 success of its exhibition, and the various exhibitors on 

 the Interest which their display evoked. C. H. L. 



ON THE INCIDENCE OF DAYLIGHT AS A 

 DETERMINING FACTOR IN BIRD- 

 MIGRATION.' 

 "T^HE existence of the phenomenon of bird migration is 

 only explicable, like that of all other phenomena of 

 life In both animal and vegetable kingdoms, by the theory 

 of natural selection. It has proved beneficial to certain 

 families of birds in the struggle for existence to wander 

 at certain times of the year in particular directions, and 

 to greater or less distances, such wanderings having led 

 them to regions which were more suitable than others for 

 feeding or breeding. On this general question there cart 

 be no difference of opinion at the present day. 



But if we leave the general problem and come to deal 

 with specific parts of It, such as the nature of the direct- 

 ing force in migration, or why for certain birds northern 

 latitudes are more suitable as breeding quarters than 

 southern, why for others eastern longitudes than western, 

 we at once enter upon questions regarding which there is 



1 Address to the Scottish Natural History Society, November 7, by Prof. 

 E. A. Schafer, F.R.S. 



