1878.1 ' JtJ [Houston and Thomson. 



chine, since a considerable portion is expended in producing what we term 

 the local action of the machine, that is local circuits in the conducting 

 masses of metal, other than the wire, composing the machine. 



The following instances of the relation between the actual work of tlie 

 circuit, and that expended in local action, will show that this latter is in 

 no wise to be neglected. In one instance an amount of power somewhat 

 more than double the total work of the circuit was thus expended. In this 

 instance also it constituted more than five times the total amount of power 

 utilized in the arc for the production of light. In another instance it con- 

 stituted less than one-third the total work of the circuit, and somewhat 

 more than one-half the work in the arc. 



Of course work expended in local action is simply thrown away, since it 

 adds only to the heating of the machine. And since the latter increases its 

 electrical resistance, it is doubly injurious. 



The local action of dynamo-electric machines is analogous to the local 

 action of a battery, and is equally injurious in its effects upon the availa- 

 ble current. 



Again, in regard to the internal work of a machine, since all this is event- 

 ually reduced to heat in the machine, the temperature during running must 

 continually rise until the loss by radiation and convection into the sur- 

 rounding air, are eventually equal the production, and the machine will at 

 last acquire a constant temperature. This temperature, however, will differ 

 in different machines according to their construction, and to the power ex- 

 pended in producing the internal work, being, of course, higher when the 

 power expended in producing the internal work is proportionally high. 



If therefore a machine during running acquires a high temperature when 

 a proper external resistance is employed, its efficiency will be low. But it 

 should not be supposed that because a machine when run without external 

 resistance, that is on short circuit, heats rapidly, that inefficiency is shown 

 thereby. On the contrary, should a machine remain comparatively cool 

 when a proper external resistance is employed, and heat greatly, when put 

 on short circuit, these conditions should be regarded as an index of its 

 efficiency. 



As a rule the internal resistance of Dynamo Electric Machines is so low 

 that to replace them by a battery, the latter, to possess an equal internal re- 

 sistance, would have to be made of very large dimensions, so that the 

 efficiency of Dynamo-Electric Machines, cannot be stated in terms of bat- 

 tery cells as ordinarily constructed. 



In regard to the second division, viz., the external work of the machine, 

 this may be applied in the production of light, heat, electrolysis, magnet- 

 ism, &c. 



Where it is desired to produce light, the external resistance is generally 

 that of an arc formed between two carbon electrodes ; the resistance of the 

 arc is therefore an important factor in determining the efficiency. To re- 

 alize the greatest economy, the resistance of the arc should be low, but 

 nevertheless should constitute the greater part of the entire circuit resis- 

 tance 



