38 THE ROYAL SOCIETY OF CANADA 



Work on the relative permeability of fabrics to helium and hydro- 

 gen has been carried out by Barr^ who made careful measurements in 

 1915. The amount of helium at his disposal was limited, the total 

 being about 310 ces. The area of the test pieces used was 25 square 

 centimetres; and measurements were made employing the volume- 

 loss method. 



Experiments on the permeability of fabrics to helium and to 

 hydrogen have recently been carried out at the United States Bureau 

 of Standards. While no account of this work has yet appeared in 

 print, it is understood that the results obtained are in close agreement 

 with those given in this paper. 



For the present work the area of the test piece used was 500 

 square centimetres, and a supply of helium of practically 100 per cent 

 purity was available. 



Definition of Permeability 



The British practice is to express permeability as the volume in 

 litres of dry gas at 15-5°C. and 760 mm. which leaks through one 

 square metre of fabric in 24 hours. Permeabilities are expressed in 

 this paper according to this definition. Some experiments, however, 

 express the volume in litres at 0°C. and 760 mm. In British practice 

 the temperature of the fabric is usually 15'5°C. 



The United States Bureau of Standards on the other hand, 

 expresses permeabilities in litres of dry gas at 0° and 760 mm. passing 

 through one square metre in 24 hours. The fabric is maintained at 

 25°C. during the period of test. - 



The essential requirements for the determination of the leakage 

 of a gas through a fabric are (i) an apparatus — usually called a per- 

 meameter — to hold a sheet of fabric so that air may be passed over 

 one surface at a definite rate while the ^ gas passes over the other 

 surface, (ii) a means of determining the amount of this gas which 

 diffuses through the fabric into the air. 



In these tests a Shakespear permeameter was used and two 

 methods were employed to determine the amount of gas in the air, 

 (i) by using a Jamin Interferometer, (ii) by using a Katharometer. 

 The permeameter is described below under "Description of Appara- 

 tus," while the use of the Interferometer and Katharometer is de- 

 tailed under "Measurement of Permeability." 



1 Barr. Loc. cit. 



2 Tech. Paper. No. 113. 



