CONTINUOUS-CURRENT DYNAMOS. 151 



attention has been paid to the over-all efficiency of the com- 

 bined steam-engine and dynamo, or the ratio which the 

 useful output at the terminals of the dynamo bears to the 

 horse-power given out by the steam in the engine cylinders 

 as shown by indicator diagrams. Two well-authen- 

 ticated instances may be cited to show the present pitch of 

 excellence. In a high-speed engine indicating 300 horse- 

 power and directly coupled to a dynamo having an output 

 of 1500 amperes at 120 volts, tests made at the Naval 

 Exhibition of 1891 in London showed an over-all efficiency 

 of 84 per cent., and on the assumption that the brake H.P. 

 of the engine was 90 per cent, of the indicated H.P., an effici- 

 ency is found for the dynamo of 93*5. With a similar 

 engine of 200 I. H.P. coupled to a dynamo for 125 volts and 

 1025 amperes at 350 revolutions per minute, an over-all 

 efficiency of 85-6 has been obtained, the dynamo alone 

 having the remarkable efficiency of 95*6 per cent. On the 

 Continent, where large multi-polar machines running at 

 considerably lower speeds are largely used, the over-all 

 efficiency of the combination is generally about 82 per cent., 

 this somewhat lower figure being accounted for partly by 

 the slower speed and partly by the fact that the numerous 

 poles demand a greater expenditure of exciting energy than 

 would be the case if their lines were concentrated into a 

 single magnetic circuit. 



Finally, it is worth remarking that such high efficiencies 

 as have been mentioned above are not obtained at the ex- 

 pense of the all-important quality of durability. It is often 

 said that an extra 1 per cent, in the efficiency is dearly 

 bought at the sacrifice of strength in some part of the 

 machine's frame. But this reasoning, true though it is in 

 many ways, loses much of its force when applied to the 

 case of dynamos, and for a reason which is easily explicable. 

 All the electrical and magnetic losses manifest themselves 

 in the end as heat, and so tend to raise the working tem- 

 perature of the dynamo : now one great guarantee that a 

 machine will be durable is that its temperature remains low, 

 so that its insulation is not likely to deteriorate from the 

 effects of heat even after many years of hard work. Hence, 



