128 THE DESIGN OF STATIC TRANSFORMERS 



of dynamo-electric machinery, there is still so little apprecia- 

 tion of the necessity for taking many details into account, if 

 tests on insulating materials are to be of thorough value. 



" The samples of insulating materials were exposed to various 

 temperatures for from nine to twelve months, and both reports 

 are in agreement in recording that none of the materials were 

 suitable for withstanding a temperature of 125 C. for this length 

 of time. Even abestos and micanite suffered at this tempera- 

 ture, the latter in consequence of the charring of the 

 material with which the component scales of the mica were 

 cemented together. 



" This experience with high temperatures further demonstrates 

 the wisdom of not at present exceeding the customary tempera- 

 ture standards. If a thermometrically-determined temperature 

 increase of 40 C. is permitted, this will frequently mean an 

 ultimate temperature of 70 C. as thennoinetrically determined, 

 and of fully 100 C. as determined from resistance measurements. 

 Still higher actual temperatures will occur at inaccessible parts 

 of the machine, notably in those portions of the field coils most 

 remote from any duct or surface. Hence, permitting a thermo- 

 metric rise of even 40 C. above the surrounding temperature, 

 brings us nearly to the ultimate temperature where the tests of 

 Messrs. Crompton & Co. and of Messrs. Siemens Bros, 

 indicate that there are few, if any, otherwise- suitable materials 

 which do not deteriorate considerably in a year or so. It is to 

 be hoped that progress in the production of suitable insulating 

 materials will be stimulated by the report of the Engineering 

 Standards Committee, as materials which will permit of higher 

 temperatures will be of great value to designers and manu- 

 facturers. The present, however, is no time for increasing the 

 standard temperature rise. Even when apparently-suitable 

 materials are on the market, a few years must elapse before 

 their suitability can be demonstrated in actual practice. 



" It required several years of actual service to demonstrate 

 that the insulating materials formerly employed in railway 



