AFTER enumerating the fundamental qualities that make an engine indicator successful, the authors classify existing indicators and discuss a few of the more successful ones.
Then is introduced the new electrical indicator, which makes its record from variation between the resistances of two carbon-piles which form the branches of a Wheatstone bridge.
The pressure element is a thin diaphragm flush with the inner walls of the combustion-chamber. This is connected by an invar rod to a cantilever spring, the displacement of which reduces the resistance in one carbon-pile and increases the resistance in the other.
The diagram is recorded by either an ordinary oscillograph or a portable cathode-ray oscillograph. Diagrams taken with this indicator are presented to illustrate its performance under varied conditions. A natural frequency of more than 3000 cycles per second makes possible the faithful recording of phenomena having frequencies as high as 800 cycles per second.