ON GASEOUS EXPLOSIONS. 197 
The position of the points A and B on the PV diagram gives no 
indication of the temperature at A or B. If the temperature at one 
of the points, however, is known, then the temperature at the second 
point can be calculated from the relation 
Se ee a ee te) 
This relation expresses the characteristic equation for gases, and is 
quite independent of the specific heat of the gases concerned. It 
applies to all positions of the state-point in the PV diagram provided 
that the following two conditions are satisfied :— 
Condition 1. That there is no change in density of the gas such as may be 
produced by some change in its chemical constitution. ’ 
’ x 2. That the weight of the working agent during the change of state 
from A to B is constant. 
It is fundamentally important, therefore, to be able to measure 
by direct observation the temperature corresponding to at least one 
position of the state-point in the diagram, because by means of this 
temperature and the relation expressed in equation (4) the temperature 
corresponding to any other position of the state-point in the diagram 
can be calculated, providing always that the conditions 1 and 2 are not 
violated during the change of state. If the first condition is violated 
there is a small change of volume caused by chemical action as the 
state-point moves from A to B, and in order to calculate the magnitude 
of this change it is necessary to have a chemical analysis of the 
gas before and after chemical action. When these analyses are known 
a correction can be made and equation (4) can still be applied to calcu- 
late the temperature. This kind of action has to be reckoned with, 
for example, if the state-point A is on the compression-curve of a gas- 
engine and the state-point B is on the expansion-curve. 
The earlier part of this report shows that the Committee have given 
a good deal of consideration to the subject of the direct measurement 
of temperature, and that individual members have worked at the 
problem successfully. Examples have been given earlier in the report 
of methods which have been applied and are being used in the 
researches which are now being carried out. This example shows how 
the equation (4) is used to calculate the temperature for different 
positions of the state-point B from observations of a single tempera- 
ture. The single temperature which it is most useful to know is the 
suction-temperature, and this may be defined as the temperature of the 
charge in the cylinder just after the admission valve is closed. There 
is then a definite weight of charge in the cylinder at a definite 
pressure and volume, and at a definite temperature. Allowing for 
the chemical contraction, equation (4) can be applied along the expan- 
sion-curve. 
The Committee have examined into the question of leak of charge, 
and have come to the conclusion that in most cases in a modern 
engine it is a negligible amount when proper precautions are taken. 
