HEAT FROM SMALL CYLINDERS IN A STREAM OF FLUID. 
431 
Table (ii -Values of the Correction Factor e. 
Wire. 
6-mil. 
3-mil. 
1-mil. 
cm. 
1 = 10-0 
•0050 
•0047 
•0051 
lo= 2-0 
l = 2-0 
•092 
•041 
•026 
lo= 0-5 
A convenient form of fork suitable for holding in position the anemometer wires, and offering a 
minimum of disturbance to the flow of air in its neighbourhood, is illustrated in fig. (a) of Diagram III. 
Fastened to a block of ebonite are the two arms of the fork, consisting of steel strips about 5 mm. in 
width. At the end of each is soldered a small brass block, drilled to receive two fine needles fastened 
about 1 cm. apart. Threaded through the eyes of these two needles is a 3-mil platinum wire, having its 
extremities firmly clamped in the brass block just mentioned. The anemometer-wire is held by these 
two loops as indicated in the figure and by this means is kept under tension by the elastic support, and is 
protected from accidental damage by the two needles on either side of each end. The tension is adjusted 
by a fine thread carried down from each of the brass blocks to an adjustable screw in the centre of the 
ebonite block; this thread is also effective in preventing lateral vibrations of the fork. Carried up from 
each end of the ebonite block are two thin steel strips crossing each other to the opposite arm of the fork, 
insulated from each other and also from the fork by means of thin mica strip. These strips serve to brace 
the fork and at the same time serve as potential leads. At each extremity is soldered a small brass block 
drilled to hold a fine needle, at the extremity of which is soldered a thin copper wire. The 1-mil platinum 
potential terminals fused to the anemometer-wire are carried to these copper wires to which they are easily 
soldered. The complete apparatus is illustrated photographically in Plate 8 (c), mounted on a micrometer 
screw for measuring rapid gradients of turbulent flow. In the experiments previously referred to in 
Section 17 it was found possible to resolve a gradient in which the velocity changed by 5 cm./sec. over a 
distance of 1/10 mm. 
In fig. ( b ) of Diagram III. are drawn the connections which were found convenient in practice. The 
resistances a and b were made equal and about 500 ohms while a and ft were adjusted to equality at about 
250 ohms. In order to protect the anemometer-wire from accidently burning out, a key Kj was inserted 
by means of which it was automatically short-circuited; a double-contact key K 2 was inserted in the 
galvanometer circuit in such a way that contact was first made through a high resistance for preliminary 
adjustments; it was also found convenient to connect the galvanometer to an adjustable shunt. The 
resistance B was constructed of No. 23 B. and S. manganin wire wound non-inductively on an asbestos 
frame in such a manner as to dissipate a maximum amount of heat; its resistance measured between 
potential terminals soldered to the wire was adjusted to four times that of the anemometer-wire at room 
temperature. By means of a fine-adjustment rheostat R the current in the anemometer-wire could be 
adjusted until a balance was obtained on the galvanometer. It is important that the rheostat be always 
re-adjusted to the position of minimum current to avoid over-heating the wire should the velocity of the 
air-flow suddenly diminish; this may be easily accomplished by means of a spring control. 
In taking a measurement of velocity, the key Ki is pressed down and the current as read by the 
ammeter slowly increased until on pressing down the key K 2 a balance is obtained on the galvanometer. 
From the reading of the current the velocity may readily be obtained from a calibration curve 
corresponding to the formula (80), i 2 = if- + k J V. It has already been mentioned that a dynamometer 
form of instrument will, if at the same time sufficiently accurate, prove to be more suitable for the 
measurement of current in that the scale-readings will be more open at high velocities; the same result 
could also be achieved by suitably shaping the pole-pieces of the permanent magnet of a direct-current 
