Notes on Small Motors. 167 



organ blowing, but is not so applicable when a rotary 

 motion at a comparatively quick speed is desired. For 

 such a purpose a turbine appears more promising ; and it is 

 next necessary to decide which of the various forms of 

 turbine should be adopted. When used on a large scale, 

 and with an ample supply of water, the merits of outward 

 flow, parallel flow, inward flow, and reaction turbines appear 

 very nearly balanced, the first cost not differing largely, and 

 the efficiency lying between 60 and 80 per cent, in each 

 case. But when the problem is to make the most of a 

 very small stream of water at a pressure of not less than 

 50 lbs. per square inch, outward, parallel, and inward flow 

 turbines need to be made of proportions, and to run at 

 speeds which are practically objectionable. Wheels of not 

 more than 1J- inches in diameter, running at speeds of 

 considerably over 5000 revolutions a minute, do not appear 

 by any means desirable in a domestic motor, and hence we 

 are led, as a last resource, to adopt the oldest form of all — 

 the reaction wheel. With this we find it is possible to 

 combine a comparatively large radius with a jet of the 

 requisite dimensions, and thus obtain a machine the economi- 

 cal working speed of which is not impracticably high. 



The reaction wheel has been known as a motor since 

 exceedingly early times, and a rude form of it actuated by 

 steam constituted the well-known ceolipyle, or rudimentary 

 steam engine of Hero, of Alexandria, B.C. ISO. In a some- 

 what crude shape, adapted for water, it forms part of the 

 apparatus of the physical lecturer, and is known as 

 "Barker's Mill" (fig. 1). Of late years the questions of its 

 economic speed and highest efficiency have been investigated 

 mathematically and experimentally, and have led to its 

 adoption on a large scale. 



The following is a summary of our knowledge on the sub- 

 ject :— 



1. With a perfect or frictionless fluid the efficiency of the 

 reaction wheel increases toward unity as its angular velocity 

 increases without limit. 



2. In a well formed reaction wheel worked by water the 

 best speed is approximately that at which the linear velocity 

 of the jet is equal to 8 Jh where h is the head of pressure. 



3. That at this speed an efficiency of fully 75 per cent, has 

 been attained. 



4. That in order to attain a good result, sharp bends and 

 sudden changes in velocity of flow should be avoided, and 



o 



