240 DR. LOUIS VESSOT KING ON THE PROPAGATION OF SOUND IN THE FREE 
is illustrated in Plate 1 (iv.) and is described in further detail in Appendix III. The 
open network formed by the iron wire gave a thermometer of minimum heat- 
capacity and very small lag. One of these was designed to fit into the large operating- 
valve of the diaphone, while the other was inserted in the resonator a few inches 
from the diaphone-piston. The resistance thermometers were connected to form the 
adjacent arms of a Wheatstone bridge and were thus operated differentially. In 
measuring a difference of temperature during a blast a rough setting was made on 
the bridge wire in the neighbourhood of the balance point, and the final reading was 
obtained in terms of the galvanometer-deflection previously calibrated in bridge-wire 
units. As the capacity of the air-compressing machinery only allowed of a six-second 
blast per minute, it was essential that the wire of the thermometer take up its final 
Inches 12 6o 1 zFeet 
Fig. 3. Determination of acoustic output of Father Point diaphone. 
Diagram showing the position of iron-wire resistance thermometers for measuring temperature 
difference of air on high and low pressure side of diaphone piston. 
temperature in this interval; this was verified by noting that the galvanometer 
deflection attained to a steady value during the last three seconds of blast. The 
difference of temperature 0 (0 — (valve-temperature) —(resonator-temperature) ) was 
measured for a certain pressure and air consumption while the diaphone was emitting 
a note ; the piston of the diaphone was then stopped and so adjusted that for the 
same pressures the air consumption remained the same as before, while the 
temperature difference O' was measured under the new conditions. It is a 
well-known thermodynamical principle, experimentally tested in the celebrated Joule- 
Thomson porous-plug experiment,* that when air under pressure is allowed to escape 
and no external work is done, there is no difference of temperature except for the 
small Joule-Thomson effect arising from the work done in separating molecules under 
* Joule and Thomson, “On the Thermal Effects of Fluids in Motion”; Kelvin, ‘Collected Works,’ 
vol. I., p. 333. 
