469 



case, because of the strong lateral pressure on tlie axis during the 

 rapid motion. The rapport between the vertical speed F,,, and the 

 number of revolutions .V can be expressed by the relation 



r„ = A -{- li X 

 in which .1 and B are constant. In the case V„ is smaller than A 

 the anemometer stands still. Kurther it would already be aditïiculty 

 to obtain a pei-manent vertical position of the axis. 



In the insti-ument the description of which follows and which we 

 have called scmubueter the foresaid disadvantages of the vertical 

 anemometer have been avoided without the advantages having l)een 

 abandoned. Therefore electric currents are measured which are 

 generated in a dynamo armature, mounted on an axis which has 

 the direction of the line of displacement of the machine instead of 

 being kept vertical. This directing as well as the rotation of the 

 axis are obtained by an airscrew fixed on the hind part of the axis, 

 which is made to rotate by reaction during the rapid motion through 

 the air, and thus gives the correct position to the well-poised axis. 

 Therefore it was practically sufficient to fix the axis pivoting around 

 a line perpendicular to the body of the machine, whicli line nnist 

 be horizontal when the aeroplane lies tlat in a lateral sense. This 

 simplification was applied for the advantage in the construction 

 only. In what follows this line is always dealt with as a horizontal 

 one, which does not introduce an appreciable error. 



The total speed of the machine Vt can, in perfect anology to 

 what has been said about the vertical velocity be represented by 



Vt = A -{- BN 



In this case the influence of the constant of friction A is much 

 smaller because Vi is always considerable, so that when the axis 

 is made to run lightly between ballbearings the number of revo- 

 lutions N is approximately proportional to Vt . 



In order to generate a current which offers a measure, not of 

 the total speed with respect to the air, but of the vertical component 

 of that quantity, T,. = Vt sin a (fig. 1), the magnetic field in which 



the dynamo-coil rotates is dressed in 

 a special way, viz. in such a way 

 that the direction of the field, which 

 we suppose to be homogeneous for 

 the sake of simplicity, is always kept 

 horizontal and besides that parallel 

 to the vertical plane through the 

 axis (jf rotation. 



3J* 



Fieri 



