Bauhaus-Universität Weimar

The velocity of the pulse wave in man in relation to age as measured by the hot-wire sphygmograph
Bramwell, J. Crighton Hill
The Wheatstone bridge employed was a special one made by Messrs. 
W. G. Pye, of Cambridge. A dial resistance is provided for the battery 
circuit, to enable adjustments of the current through the platinum wire to 
be made. Sufficient current is employed to heat the wire to a temperature 
just below red heat. A dial resistance is provided in series with, and a dial 
shunt for, the galvanometer. A spiral ratio resistance (with scales) forms 
two arms of the bridge, a dial resistance the third arm, and the hot wire the 
fourth. The resistance of the third arm is usually made about equal to that 
of the hot wire. Due precautions must be taken in connecting up, in getting 
the balance, and in manipulating the bridge, in order to avoid breaking 
the galvanometer string. Three to six accumulators are required. 
(d) Connecting pipes. The pipe conveying the pulsations from the 
artery to the hot wire should be as short as possible, for the following reason. 
A pipe of length l, closed at one end, acts as an organ pipe, and resonates 
with a frequency of a/4l, where a is the velocity of sound. The air, therefore, 
in a pipe two metres in length has a natural frequency of oscillation of 
42* per second, in a pipe one metre long a frequency of 85 per second. Hence, 
if we wish to avoid contamination of our records with vibration of the air 
in the pipe, it is necessary to use short pipes. 
A set of records was made by gently tapping, and so setting up 
vibrations in the air inside, a series of rubber pipes of different lengths. 
The pipes consisted of pressure tubing and were 3-47, 2-54, and 1-74 metres 
long respectively. The hot wire was near the open end of the pipe, and 
the other end was closed. The fundamental frequenciesf of these pipes 
should be 24-5, 33-5 and 49-0 per second respectively. We found by 
measurement that the record showed vibrations at rather less than double 
these rates, viz., at 41, 54 and 82 respectively. These frequencies are exactly 
in the inverse ratios of the length of the pipes, as they should be. The hot 
wire we should expect to respond twice to each complete oscillation of the 
air, once as the air goes in, once as it goes out. The reason why the 
frequencies observed are not exactly double the calculated ones is the narrow 
gauge of the pipe employed (about 4 mm.). It is known that the velocity 
of sound in a narrow pipe is less than in a wide one, and in the pipes 
employed it would appear to be about 280 metres per second. Thus, in 
employing a narrow rubber pipe of length 1 metre, with one end open, for 
recording the pulse, we have to expect vibrations of the air of frequency 
about 70/I per second. Such artificial vibrations might be expected to 
interfere with, and to distort, the record of the vibrations of the arteries, 
which are often themselves fairly rapid. In the case of an instrument like 
the polygraph, where the transmitting pipe is closed at both ends, the wave- 
* Assuming the velocity of sound to be 340 metres per second, as in the open air. 
t Calculated from the expression a/4l, and assuming a, tho velocity of sound, to be 
340 metros per second.


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