Directional Hot-Wire Anemometer. 



659 



necessarily relatively small — of the order one-thousandth, 

 at most— compared with « and /3, and as moreover no 



measurable alteration in their respective values accompanied 

 an alteration of G from to go, we have 



fdy \ {(b/3-au) (a + /3) } x 



\dO A { G(a + 6 + « + £) + (6 + a) (a + 0) } 2 ' 



and A?/ the alteration in the current through the leading 

 anemometer wire accompanying an alteration of the galvano- 

 meter resistance from an infinite value to the value G is 

 given by 



(A//) --(6ft-qa)(a + ft)&' 



V J)x (a + ft + a + £){G(a+& + a + £) + (& + *)(a + £)} 

 and is readily seen to be very small for all values of G, under 

 the conditions of the present experiments. It may also be 

 readily shown that under the conditions of the present 

 experiments, in which a =^1000, £=1000, and a-*-b is at 

 the most of the order 0*1 ohm, the value of the galvanometer 

 current alters only between the limits and 10" 4 ./' when the 

 galvanometer resistance is altered from cc to zero, so that, 

 to within 1 part in 10,000 at least, the current passing- 

 through both anemometer wires is the same. The values 

 of the impressed velocities at which the respective maximum 



