666 



NATURE 



[August 28, 1913 



conference on electrical units and standards 

 (1908). 



For direct-current measurements in electricity 

 the three fundamental units — the ohm, the ampere, 

 and the volt — are sufficient ; but the Reichsanstalt 

 experiments have contributed to a great extent 

 to the improvement and simplification of methods 

 of measurement. It was this institution that 

 helped largely in the development of the potentio- 

 meter, which, in 'conjunction with the standard cell, 

 forms the real basis of very many electrical 

 measurements. 



Mention should also be made of the "artificial 

 loading " method introduced by the Reichsanstalt 

 (separate circuit for current and pressure), which 

 enables tests to be carried out, with but a small 

 expenditure of electrical energy, on apparatus in- 

 tended for the measurement of outputs up to many 

 thousand kilowatts. It was also the Reichsanstalt 

 which introduced the optical method of measuring 

 current densities by means of the optical pyro- 

 meter. 



Considerable interest attaches to the testing of 

 sheet-iron employed in dynamo and transformer 

 construction, and endeavours have repeatedly been 

 made to keep down the energy losses in the 

 material while augmenting the permeability (mag- 

 netisability) of the iron as much as possible. The 

 Reichsanstalt was the first to suggest the use of 

 iron alloyed with silicon for this purpose ; and the 

 researches of Barrett, Brown, and Hadfield appear 

 to show the great advantage of this alloyed iron, 

 which results in the reduction of eddy currents. 



The optical work of the Reichsanstalt is not dis- 

 cussed quite so fully as that of the other branches. 

 After a mention of pre-existing units of light, a 

 space is devoted to the amyl acetate lamp of 

 Hefner (briefly, the Hefner lamp) as a photometric 

 standard. Whilst recognising its many advan- 

 tages the Reichsanstalt has not lost sight of its 

 deficiencies, and for this reason has always en- 

 deavoured to establish a measure of light to satisfy 

 the broadest requirements of scientific and technical 

 practice. Successful experiments have been con- 

 ducted to secure a constant radiation of light from 

 incandescent platinum with the aid of the bolo- 

 meter, and tests now in hand justify the hope of 

 arriving shortly at a mode of realisation of the 

 "black body" which will fulfil all requirements in 

 regard to accuracy and trustworthiness. Through 

 the labours of the Reichsanstalt Germany was the 

 first country to possess a generally recognised, 

 accurately investigated unit of light. 



The Charlottenburg establishment has also kept 

 in close touch with practical requirements con- 

 nected with saccharimetrical work. For ascer- 

 taining the value of sugar use is made of its 

 rotation relative to the plane of polarisation, the 

 Germans (and many other countries) using the 

 Ventzke scale of divisions for the saccharimeters. 

 The hundred point of this scale is defined by the 

 rotation of a standard sugar solution (26 grammes 

 in 100 c.c. of water at 2o°C.) in a 20 cm. tube. 

 For checking the readings of this apparatus, a 

 quartz plate ground perpendicular to the axis is 

 NO. 2287, VOL. 91] 



employed. Extensive experiments had to be made 

 on the rotation of the pure sugar at the concen- 

 tration of the standard solution, in order to arrive 

 at a basis for test purposes. Great accuracy is 

 necessary, as it is estimated that an error of o"i 

 per cent, would make a difference in the sale value 

 of the annual production of sugar in Germany of 

 about 25,000?. The Reichsanstalt has, in addition, 

 introduced the Abbe refractometer for determining 

 the percentage of solids or of dry substance in 

 connection with the impure sugar solutions to 

 be investigated in the course of manufacture. 

 Particulars are given of the method employed. 

 Experiments have also been undertaken on the 

 refractivity of other substances with the Abbe- 

 Fizeau dilatometer, such as the refraction of differ- 

 ent gases (air, H, N, and He) at room temperature 

 and at very low temperatures ; in addition, accur- 

 ate measurements have been made on the refrac- 

 tive power of quartz and fluorspar — substances 

 of such great importance in radiation measure- 

 ments. 



A series of optical experiments on metals have 

 been made, yielding important results. The re- 

 flecting power was first determined by measuring 

 the quantity of light reflected nearly perpendicular 

 (to within \°) to the surface, for light of different 

 wave-lengths, not only for the visible part of the 

 spectrum, but also for ultra-violet (to wave- 

 lengths of 0'25/i) and for ultra-red rays (to wave- 

 lengths of 1'5/x). In the visible range the work 

 was carried out with the spectrophotometer, and 

 in the invisible range with a Rubens thermopile. 

 In addition to pure metals the technically im- 

 portant mirror-alloys were investigated, and, for 

 the visible range, glass mirrors coated with silver 

 and mercury amalgam. It is interesting to note 

 that silver, which in the visible spectrum reflects 

 better (viz. 90-95 per cent.) than all other metals, 

 reflects much less than all other metals in an 

 ultra-violet region (between 0*25 and o"3/x), 

 namely, only about 4 per cent. — or less even than 

 a quartz surface. 



An apparatus has been constructed by the 

 Reichsanstalt for producing sharp interference 

 bands, and having a high capacity. Its main 

 constituent is a plane-parallel glass strip into 

 which the light to be tested is transmitted in 

 such manner as to fall on the bounding planes 

 near to the angle of total reflection, thus emerging 

 striated. With this apparatus a number of 

 spectrum lines, particularly the mercury lines, 

 have been tested as regards their structure and the 

 presence of accompanying lines ("satellites"). 



In conclusion mention should be made of a 

 series of experiments relative to the luminous 

 phenomena in highly evacuated Geissler tubes. 

 A new kind of ray, similar to the kathode rays, 

 was discovered which was emitted from the anode 

 under certain conditions. These anode rays, which 

 are emitted in particular from hot salt anodes, 

 show magnetic and electric deflection like the 

 kathode rays and the Doppler effect (displacement 

 of the lines in the spectrum). They were conceived 

 as being positively charged metallic atoms of the 



