as a Generator and as a Motor. 21 



numbers who hold clear and correct views on the subject of 

 electric generators, it is by no means easy to find definite and 

 correct explanations of what is right and what wrong in regard 

 to the allied subject of electric motors. 



And on no point does there appear to be greater uncertainty 

 than on that of the analogies and the differences between the 

 two machines. For instance, while it has been proved beyond 

 question that in dynamos the most economical and best working 

 conditions are obtained by employing field-magnets of great 

 magnetic strength, and armatures which are relatively very 

 weak magnetically, the only definite assertion within the 

 writer's knowledge given with authority on this point, asserts 

 exactly the contrary to be the case with motors, viz. that the 

 fields should be relatively much lighter and weaker than the 

 armatures*. 



Again, what can be more striking than the great and known 

 difference between dynamos and motors in respect to the very 

 important matter of efficiency ! Almost any modern generator 

 will give a return in the form of electric energy of 90 per 

 cent, of the mechanical energy expended in it, and a useful 

 yield of over 80 per cent, in its external circuit. In the case 

 of the best generators this is a considerable understatement of 

 the working conditions. With electric motors, on the other 

 hand, one has to be contented with a much lower efficiency. 

 The energy supplied to them is severely taxed in the process 

 of conversion. Motors frequently yield, as mechanical energy 

 in a useful and available form, only 30 per cent., and seldom 

 more than 60 per cent., of the electric energy absorbed. 



The reversibility of the dynamo, or its ability to work either 

 as generator or motor, has been spoken of as one of the 

 most important discoveries of late years (the great physicist 

 credited with the expression of this opinion having apparently 

 overlooked the fact that Pacinotti made his famous machine 

 as a motor in the first place). But the fact that a given 

 machine which will work well and efficiently as a dynamo, 

 will not work so economically when its action is reversed 

 and it is made to act as a motor, is a proof that the reversal 

 is accompanied by some radical change in its action. This 

 difference, where it is noticed at all, is generally sought to be 

 explained by an assertion or a supposition that having been de- 

 signed and made for one purpose, it was therefore not suitable 

 for the other. No satisfactory explanation, as far as the writer 

 is aware, has been given of the cause of this lower efficiency 



* Profs. Ayrton and Perry, "Electromotors and their Government," 

 Journ. Soc. Teleg. Eng. and Electricians, March 1883. 



