DEVELOPMENTS IN COMMUNICATION MATERIALS 257 



much less material is required to obtain the desired result that there is a 

 net saving in cost of manufacture. The true criterion of relative 

 economy, however, takes into account not alone the cost of manufac- 

 ture, but the serviceability of the device throughout its operating life. 

 Hence the designer, if he be free to decide on purely engineering 

 grounds, will make his decision as to the best materials to use on the 

 basis of the lowest annual charge over a period of years, thus taking 

 into account the important item of maintenance cost. 



