9
high musical note best described as a very shrill squeak and the noise made by the
air leaving the mouth of the whistle. To apply the test the whistle is sounded
and the length shortened until a point is reached when the squeak becomes
inaudible. With a little practice this can be easily done ; the length of the whistle
is then measured by inserting a wedge-shaped ivory scale between a flange fixed
to the plug itself and a flange on the whistle : the numbers engraved on the scale
giving the length of the whistle in millimetres. The following table gives the
number of vibrations per second of a column of air of different lengths, at a
temperature of 15° C.
Length of Column of air
Number of complete
Number of complete
in millimetres.
vibrations per second
vibrations per second
by calculation.
by experiment.
10
85,000
1-2
70,830
1-4
60,710
1-5
56,670
1-6
53,130
1-8
47,220
20
42,500
25
34,000
3 0
28,330
3-5
24,290
40
21,250
5 0
17,000
60
14,170
7 0
12,140
80
10,630
90
9,440
100
8,500
The numbers in the second column are found by the following formula
n=no. of vibrations per second.
» = velocity of sound taken at 340 metres per second.
X = wave length.
The figures in the next column will be inserted when the experiments are
completed. Care is taken to make all the whistles as nearly identical as possible.
Price complete in pocket case. £15 0
Reaction time to Sound and Sight.
This instrument is for measuring the interval of time between a signal and the
depression of a key by the person observing the signal. The person sits down and
rests his hand on a slab with his finger on a key. On hearing or seeing the signal,
as the case may be, he presses the key as quickly as possible. The time is measured
by the distance through which a rod has fallen and its amount is read off directly
from graduations on the rod itself in hundredths of a second. To use the
ST TIBB S BOW, CAMBRIDGE, ENGLAND.