188 REPORTS ON THE STATE OF SCIENCE.—1914. 
Fig. 9 shows temperature-curves for the compression stroke calcu- 
lated in this way, both for trials 72 and 73. Trial 73 was run at about 
200 r.p.m. 
The constant, however, cannot be applied during the whole eycle, 
because, although the weight of the charge remains the same, assum- 
ing that there is no leak, yet the volume corresponding to this weight 
is slightly different after the explosion has taken place owing to the 
contraction due to the chemical rearrangement of the constituents. 
The chemical contraction is calculated from the analyses of the gases. 
In the gas used in the experiments referred to the contraction 
amounted to 3°14 per cent. The effect of this is to change the gas 
constant for all points along the expansion-curve from 0°01616 to 
0°01565. moot 
The curve, fig. 10, shows the temperatures calculated along the 
expansion-curves for trials 72 and 73. Ly: 
When applying this method of taking the temperatures the governor 
should be put out of action, so that there shall be no change in the 
rate of the supply of gas which will produce a disturbance of the 
temperature in the cycle. Any disturbance produced in a particular 
cycle causes a temperature wave through a long series of succeeding 
cycles. In practice the gas-engine can be run without any difficulty 
without the governor if the engine is coupled to a generator, because 
the generator automatically settles down to the speed corresponding 
to the power applied to it, and by regulating the resistance of the 
armature or the fields, or both, the desired speed can be maintained 
for long periods. A special switch-board and a resistance-board have 
been designed for the engine at the City and Guilds (Engineering) 
College for the purpose of controlling the generator. 
Method of Measuring the Temperature of the Charge by means of a 
Thermo-couple. 
The second method of measuring the temperature of the charge 
in the cylinder is by means of a couple. This method has been 
developed by Dr. Coker and Mr. Scoble at the Technical College, Fins- 
bury. It was found that alloys of platinum with rhodium and iridium 
respectively were able to withstand the temperature of explosion 
near the walls of the cylinder for some hours or even days when 
made into thermo-couples ;5355 to z5859 1 an inch thick, provided 
The general relation between electromotive force and temperature 
found for one of the couples used is 
E (microvolts)= —174 + 7.60757 — 0.001673T?. 
The general arrangement of the apparatus is shown in fig. 11. 
The battery B and resistances R, and R, are arranged in circuit 
so that the fall of potential between the extreme points of a bridge- 
