JANUARY 3, 1907] 
NATURE 
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RECENT WORK OF THE AMERICAN 
BUREAU OF STANDARDS. 
Part i. of vol. ii. of the Bulletin of the Bureau of 
Standards of the United States contains five papers. 
The first of these is by Mr. Hyde, on Talbot’s law as 
applied to the rotating sectored disc. The law is stated 
by Helmholtz as follows :—‘‘ If any part of the retina is 
excited with intermittent light recurring periodically and 
regularly in the same way, and if the period is sufficiently 
short, a continuous impression will result, which is the 
same as that which would result if the total light received 
during each period were uniformly distributed throughout 
the whole period.”’ 
Much experimental work has been done on the subject 
leading to somewhat conflicting results. After a theoretical 
discussion Mr. Hyde describes his own experiments, from 
» which he concludes :— 
(1) Talbot’s law is verified for white light for all total 
angular openings from 10° to 288° to within a possible 
error of 0-3 per cent. 
(2) The observed deviations from the law for red, green, 
and blue light are of the same order as those for white 
light. 
The two papers by Dr. Guthe and Mr. Rosa respectively 
deal with a new determination of the E.M.F. of the 
Weston and Clark cells by a Gray electrodynamometer. 
The construction of the instrument and the measurement of 
its exact dimensions are described in great detail by Dr. 
Guthe in the first paper, while in the second Mr. Rosa 
gives the complete theory of the dynamometer employed, and 
discusses the effects of various errors, such as inaccuracy 
in the measurement of the dimensions of the coils, irregu- 
larities in their winding, and the effect of the opening in 
the fixed coil through which the suspension of the movable 
coil passes. The chief results of the work are as 
follows :— 
The E.M.F. of Guthe’s ‘‘ reference standard ’’ cadmium 
cell, No. 813, at 21° C.=1-01884 volts, or only two parts 
in 100,000 higher than the value given in its Reichsanstalt 
certificate. The electrochemical equivalent of silver, deter- 
mined by Guthe in 1904 (Bull. Bur. Stand., vol. i., part i.) 
in terms of a Weston cell, becomes, when re-calculated to 
absolute measure, 1-11773 mg. per coulomb, when the 
porous-pot form of coulometer is used. 
In the next paper Prof. J. G. Coffin discusses the con- 
struction and calculation of inductance standards for Clark 
University and for the Bureau of Standards. The paper 
gives in great detail an account of the successive oper- 
ations. One of the most interesting of these is the use 
for the accurate grinding of the marble cylinders of an 
especially fine grinding-machine on a novel principle, which 
will grind, if required, true cylinders up to 71 cm. diameter, 
more than 3 metres long, and up to ten tons in weight. 
The construction and winding of the cylinders presented 
many of the same problems as the making of the coils 
of the electrodynamometer referred to above, the coils in 
both cases being formed of a single layer of wire divided 
into several parts highly insulated from one another. 
Following the practical details comes a long mathematical 
discussion setting forth the methods employed for calcu- 
lation to the high accuracy required of the inductances of 
the various parts, the two different formule employed 
giving, for the calculated self-inductance of one of the 
sections of the Clark University coil, values only differing 
by one part in four hundred thousand. ‘ 
The last paper, by Messrs. Hyde and Brooks, is on an 
efficiency meter for electric incandescent lamps. It consists 
of an attachment to a photometer on which a scale of 
watts-per-candle can be arranged,. so that the “‘ efficiency ”’ 
of the lamp to be tested can be directly read off in a very 
simple manner without calculation. The essential feature 
consists in the right design to give the desired result of 
a rheostat in the lamp-circuit, operated by the sliding of 
one of the photometer carriages. A table of results shows 
that for a nominal sixteen candle-power lamp over a range 
of from ten to twenty candles, with accompanying vari- 
ation of watts-per-candle of from 4-8 to 2-4, the value for 
the efficiency as obtained by the instrument and that deter- 
mined independently by check instruments agreed every- 
where to less than 1 per cent. The value of the arrange- 
NO. 1940, VOL. 75] 
ment lies in, the extreme rapidity with which a definite 
criterion for a large number of specimen lamps can be 
obtained without troublesome arithmetic. 
In part ii. of vol. ii. the first paper, by Mr. Rosa, deals 
with the calculation of the inductance of single-layer coils. 
The different types of ‘‘ summation ’’ and “ current-sheet ” 
formule due to Rayleigh, Coffin, and Lorenz, and their 
suitability for use with coils of different shapes, are dis- 
cussed at length. Examples are also given showing the 
degree of concordance obtained in definite cases. ‘lables 
of general application are calculated, from which the 
correction term for coils of varying number of turns, made 
of wire of different diameters, may readily be obtained. 
The paper by Mr. H. C. Dickinson is entitled ‘* Heat 
Treatment of High-temperature Mercurial Thermometers.” 
It begins by recapitulating the methods of construction of 
high-range thermometers and the properties and suitability 
for different ranges of the various high-temperature glasses 
employed. Tables of the departure of the natural scale of 
16"" and 59”! glasses from the gas scale are given up to 
temperatures of 300° C. and 500° C. respectively. The . 
author then describes his own experiments, which deal 
chiefly with the effect of initial heat treatment on various 
unannealed thermometers of the different glasses specially 
constructed for research purposes. The best methods of 
annealing such thermometers for rendering them as per- 
manent as possible in after use are described. An electric 
furnace arranged for the purpose is also shown. The 
following are the more important conclusions the 
paper :— 
(1) Jena 59’" borosilicate is the best thermometric glass 
in use for high temperatures, but it cannot be safely used 
much above 500° C. 
(2) Jena 16”’ glass can be used up to 450° C. 
(3) Every thermometer intended for use above 100° C. 
should undergo a suitable system of annealing before use. 
The annealing may be done before the thermometer is 
filled. A thorough anneal requires four to ten days at 
450° C. The anneal may be followed with advantage by 
a period of slow cooling of from three to six days. 
(4) To prevent the boiling of the mercury in a thermo- 
meter, the space above it should be filled with dry nitrogen 
or carbon dioxide, having a pressure of one atmosphere at 
300° C., of four and a half atmospheres at 450° C., and of 
twenty atmospheres at 550° C. 
Mr. Brooks in his paper describes a new potentiometer 
for E.M.F. and current measurements of intermediate 
accuracy. It is primarily intended for use in such work 
as photometry, where rapidity of reading is essential, and 
where the best deflection instruments give an accuracy, in- 
sufficient in most cases. 
The feature of the new design is that the potentiometer 
method is used to balance the bulk of the electrical quantity 
to be measured, the remainder, perhaps 1 per cent. of the 
whole, being shown by a suitable deflection instrument. 
The design of a successful deflection potentiometer presents 
several difficulties. In the present case these have been 
surmounted in a manner similar to that adopted by 
Stansfield, who was the first to use this type of instrument. 
The greatest scale error in the new instrument described is 
0-02 in 100 volts. 
In a paper on spectrum lines as light sources Mr. Bates 
discusses the structure of the sodium D lines and the 
green mercury line A=546-1 uw as sources in polariscopic 
measurements. Sodium lines obtained in different ways 
show slight differences. For intense illumination the 
author prefers sticks of pure Na,CO, fed into an oxy- 
hydrogen flame. An echelon spectroscope was used for the 
study of the lines. The position and intensity of the 
satellites of the green mercury line were accurately 
measured. The use of this line is proposed as the standard 
source for all accurate polariscopic work. A quartz rota- 
tion for wave-length 589-25 “u may be obtained by measur- 
ing the rotation for the wave-length 546-1 #u, and multi- 
plying it by the constant 0-850944. 
The paper by Mr. Nutting, on polarimetric sensibility 
and accuracy, hardly permits of useful abstraction. It 
deals with the intensity and homogeneity of sources used 
in polarimetry. The mathematical theory of the half- 
shadow polarimeter is discussed, and formule given for 
calculating the sensibility of the instrument under varying 
kc 
of 
° 
