Aug., I92o.] U . S . BUREAU OF STANDARDS NOTES. 253

were considered to be of interest to users of magnetic compasses. This paper gives a brief discussion of the principal performance characteristics of magnetic compasses togelher with a descrip- tion of some of the apparatus which is used at the Bureau for testing.

P E R M E A B I L I T Y OF R U B B E R TO GASES. TM

By J. D. Edwards and S. F. Pickering.

[ABSTRACT. I

CEICrAX~ of the factors which determine the permeability of rubber to gases have been investigated and the relative rates of penetration of a number of gases determined. The major findings may be summarized as follows:

I. The permeability of rubber compounds varies with the composition as would be expected. The aging of rubber films is. accompanied by a decrease in p,ermeability. A similar decrease may be affected by over-vulcanization. The rubber which shows a very low permeability for these reasons is. usually very nmch deteriorated and frequently brittle so that it is a disadvantage f rom the standp.o~int of gas lighters.

2. The permeability to any gas is found to be directly pro.- porticnal to its partial pressure provided the total pressure is constant. The variation of permeability with total pressure depends on the thickness of the rubber, the way in which it is supported, etc.

3. rl he permeability to hydrogen is inversely proportional to the thickness of the rubber. No other gas was tested in this respect.

4. The specific permeability to hydrogen at 25 ° C. of vul- canized rubber similar to the grade known as dental dam is about 20 × ~o -6 c.c. per minute. This value varies somewhat with the age and chemical characteristics, of the rubber.

5. The temperature coefficient of permeability is quite high. For example, in the t e s t s a t ~oo ° C., the permeability to carbon dioxide or helium was about 17 times, the rate at o ° C. ; the per- meability to hydrogen was alx)ut 22 times as great at IOO ° as at o ° C.

i~ Scientific Paper No. 387.

254 U . S . BUREAU OF STANDARDS NOTES. [J. F. [.

6. T h e relative permeabil i ty of rubber to some common gases is shown in the fol lowing summary :

R e l a t w e Gas Permeabi l i ty

I~y drogen = Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o.I6 Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.22 Argon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o.26 Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o.45 Helium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o.65 Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I .oo Carbon dioxide . . . . . . . . . . . . . . . . . . . . . . . . . 2. 9 Ammonia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.0 Methyl chloride . . . . . . . . . . . . . . . . . . . . . . . . I8.5 Ethyl chloride . . . . . . . . . . . . . . . . . . . . . . . . . 2oo.0

7. The permeabil i ty of rubber to water vapor is h i g h - - a p p r o x - imately 5o times the permeabil i ty to hydrogen. The value not having been de termined with any precision is not included in the table above.

ATOMIC T H E O R Y A N D LOW VOLTAGE ARCS IN C ~ S I U M VAPOR. TM

By Paul D. Foote and W. F. Meggers.

[ABSTRACT.]

Theoretical.--On the basis of several possible theories of a tomic s t ruc ture it is shown that the normal opera t ion of an arc below ionizat ion might result in the exci ta t ion of a single line spectrum, a single series spectrum, or a gro,up spectrum consist- ing" of certain lines of different series. Th i s latte~ conclusion follows f r o m an extension of Bohr ' s theory. T h u s if inelastic electrolfiC-atomic impact occurred result ing in the eject ion of an electron to the p th ring, the electron ifi r e tu rn ing to, the n th r ing or equil ibrium may produce any combinat ion of lines represented by inter orbit t ransi t ions within this range, the single line spectrum being a special case where p = n +I.

A simple explanat ion is offered of fluorescence phenomena in vapors of the alkali metals.

A mechanism of absorpt ion of radiat ion is described and the theory proposed by K. Compton that the ionization of an a tom below the ionization potential may be explained by absorption

~5 Scientific Paper No. 386.