184 REPORTS ON THE STATE OF SCIENCE.—1914, 
The temperature is measured, therefore, during a particular crank- 
angle determined by the setting of the contact-maker. This can 
be set, while the engine is running, to determine the make and 
break at any assigned crank-angle in the revolution. It was usually 
set so that the interval between the make and break was 5° or 109. 
In this manner the mean temperature over a small crank-angle can 
be measured at any point in the cycle, except only during the period 
of the explosions when the thermometer is withdrawn from the cylinder. 
But although there is this possibility with the method it is desirable 
to measure the temperature at a point on the cycle where the rate 
of change of temperature is at a minimum. ‘This point occurs just 
after the closing of the suction-valve. The great advantage of making 
the measurement at this point is that the thermometer is exposed to 
the incoming charge during the whole of the suction-stroke and 
therefore the thermometer-valve tends to assume the temperature of 
the charge; consequently the temperature which the small wire is set 
to measure does not differ greatly from the temperature of the metal 
in which it is mounted. This condition tends to minimise the errors 
of measurement. At any other point in the cycle the rate of change 
of temperature is greater; and the error of the measurements, there- 
fore, is likely to be greater owing to the lag of the thermometer. 
On the expansion-stroke, for example, the temperature may vary as 
much as 150° during the movement of the piston through ;4, of the 
stroke. Just after the closing of the suction-valve the variation of 
temperature during the movement of the piston through ;4, of the 
stroke is only about 20°. 
Having found the temperature at one point in the cycle, the tem- 
perature at any other point can be calculated by using the charge itself 
as the thermometric agent. The characteristic equation of the charge is 
oa constant. If, therefore, from the indicator diagram taken at 
the time the temperature was measured, the corresponding pressure 
and volume are measured, then the temperature at any other point of 
the cycle can be calculated by the aid of this constant and the pressure 
and volume scaled from the indicator diagram, allowance being made 
for chemical contraction of the charge after explosion. It is necessary 
to have accurate indicator diagrams from which to measure the 
pressure and volume for this purpose, and this has led to the develop- 
ment of an optical indicator. 
Ezample of the Application of the Method to an Engine Trial 
(72) at the City and Guilds (Engineering) College. 
The general procedure in making temperature measurements by 
this method, and with an improved optical indicator devised by Pro- 
fessor Dalby and Dr. Watson, may be illustrated by data obtained 
during a trial made at the City and Guilds (Engineering) College by 
Professor Dalby last year, a full report of which will be found in 
Note 32 communicated to the Committee. 
