30 The Contrast of Practice and Theory. 



keen eye for the false and imperfect assumptions of theory, and 

 his ability to allow for, or detect them and their consequences, 

 had only a limited range of usefulness. He did not feel as if he 

 knew how to see through the questions before him, and even 

 where he did, the two or three per cent, advantage to be gained 

 thereby seemed no great thing. As electrical engineering 

 practice, however, extends, the number of persons who have 

 got more or less accustomed to the way of handling the ques- 

 tions that arise increases, and the importance in practice of 

 those things which are left out of account in theory increases. 



As long as dynamos were toys, details about the way the 

 wires were fixed on the armature, what composition of brass or 

 bronze was used for the commutator bars, what precise con- 

 struction and arrangement of switches, &c., was most convenient, 

 and so forth, were unconsidered trifles. These things never 

 come into the theory from a scientific point of view at all ; but 

 they have all become matters of practical importance. 



The case, however, of many of these questions is very different 

 from what might be called somewhat parallel questions in the 

 case of a steam engine, in that, with regard to several of them, 

 theory has often a say in the choice of means to be adopted in 

 overcoming difficulties. Heating in certain parts of a dynamo, 

 such as the armature and horns of field magnets, can for instance 

 be overcome either by a circulation of cold air or water, or by 

 adopting particular ways of constructing those parts : theory in 

 these cases, by affording a choice of means, pointed out those to 

 be adopted in practice. 



The aid to be got from theory is, however, unequal in differ- 

 ent matters. For instance, with alternating systems of very 

 high pressure it was found that materials which prevented the 

 passage of electricity for a short time pretty well, broke down 

 after a while. It was as if the insulation of the conductors was 

 unable to stand perpetual jarring ; and special insulating com- 

 pounds have therefore to be sought out, just as special qualities 

 of iron or steel have to be used in particular parts of ordinary 

 machinery, selected on account not only of their strength, but 

 of their durability and reliability as well. 



