Theory of the Robinson Cup- Anemometer. 63 



10. To find the characteristic of the motor at any given load 

 we must eliminate and <£ between the equations 



p = ce cos <p, 

 sm a = y> 

 and E 2 = * 2 + IV + 2Ice cos (#-(/>). 



The eliminant is 



E 2 -^-IV-2Rp = 2Sv/ c V-^ 2 , ... (8) 



which is the required general relation between e and c. 



11. In the paper referred to above, Steinmetz calls equation 

 (8) the Fundamental Equation of the Synchronous Motor : the 

 equation is there developed and plotted ; results are obtained 

 directly from equation (8) , but the development is so cum- 

 brous that the writer thinks that his simple treatment may 

 benefit those interested in the subject of Alternate Current 

 Motors. 



III. Contribution to the Theory of the Robinson Cup- 

 Anemometer. By C. Chree, M.A.* 



§ 1. rilHE velocity of the wind during a hurricane is one of 

 JL those items of information that, like the speed of 

 express trains, is a source of general interest. The question of 

 the accuracy to be expected in such definite statements as that 

 the wind has blown at 60 miles an hour, though of much less 

 general interest, is still of some practical importance. The 

 usual instrument for the measurement of wind-velocities in 

 this country is the Robinsonf cup-anemometer. This consists 

 fundamentally of four hemispherical cups attached to arms, 

 inclined to each other at angles of 90° in a horizontal plane, 

 the cups moving under the influence of the wind round a 

 vertical axis. The number of revolutions of the cups in a 

 given time is the information which the instrument, if properly 

 constructed, could be relied on to give. The information 

 desired, however, is what is the time-integral of the wind's 

 velocity. To obtain this it is usually assumed that the wind's 

 velocity is deducible by multiplying that of the centres of the 

 cups by a constant " factor/' and the recording mechanism is 

 arranged so as to give the result of this multiplication. The 

 value originally proposed for this factor by Dr. Robinson was 

 3, and this is the value generally employed in reducing results 



* Communicated by the Author. 



f So called after Dr. Robinson of Armagh. 



