1911] on Rpcpnt Adranrcs in Tvrhinos. 93 



armature core and small diameters and means to resist the great 

 centrifugal forces. The dynamo was also mounted in elastic bear- 

 ings. Now that the turbine has found its most suitable field in 

 large powers, and the speed of revolution is consequently reduced, 

 elasticity in the bearings is less essential, and in large land plants and 

 in marine work rigid bearings are now universal. I have said that 

 steam behaves like an incomju'essible fluid in each turbine of the 

 series, but as it is highly elastic, its volumt; increases with the succes- 

 sion of small drops of pressure, and the turbines have to be made 

 larger and larger. This enlargement is secured by increasing the 

 height of blade, by increasing the diameter of the succeeding drums, 

 and by altering the angles and openings between the blades. All 

 three methods are generally adopted to accommodate the expanded 

 v(ilume of one hundredfold in the condensing turbine. 



Now as to the best speed of the blades. It will be easily seen 

 that in order to obtain as much power as possible from a given 

 quantity of steam, each individual row of blades must work under 

 appropriate conditions. This, as has been found by experiment, 

 requires that the velocity of the blades relatively to the guide blades 

 shall be about one half the velocity of the steam, or, more accurately, 

 equal to one half the velocity of issue from rest due to the drop of 

 pressure in guides or moving blades. The curve for efficiency in 

 relation to the velocity ratio has a fairly flat top, so that the range 

 of velocity ratio for high efficiency is wide, and the speed of the tur- 

 Ijine may be varied considei'ably about that for maxinnim efficiency 

 without materially affecting the result. 



In compound turbines the efficiency of the initial rows may be 

 generally assumed as about 65 per cent., and of the latter rows at 75 

 to 85 per cent., and, considering the whole turbine, approximately 

 75 per cent, of the energy in the steam is delivered on to the shaft. 

 The expansion curve may be expressed approximately by pv = log 

 P\IP-2i where p^ - p., is the drop in pressure across any turbine ; pv is 

 obviously not quite constant, but if a mean value is assumed the 

 error is small. The expansion curve therefore lies between the adia- 

 batic and isothermal curves foi' steam, but nearer the former, and the 

 errors in these assumptions are found by experiment to be of much 

 less importance than the errors in workmanship and imperfections of 

 materials that are unavoidable in practical mechanics. The differen- 

 tial thermal expansions of the metal of which the turbine is made are 

 the chief reason for large working clearances and loss by leakage, 

 though every available means is taken to mitigate such loss. 



In turbine design, the expression of the velocity ratio between the 

 steam and blades may be represented by the integral of the square of 

 the velocity of each row through the turbine, which is a coefficient 

 called K. If K, for instance, as usual in land turbines, equals I50,(»()U, 

 tluni we know that with a boiler pressure of 200 lb. and a good 

 vacuum the velocity ratio is 0*55, and the turbine is working close 



