3. This mixture is compressed by the piston in the cylinder to about one-fifth its original volume.
4. The mixture is heated to its kindling temperature, which is above 2000 degrees. It then burns with a sudden expansion, which drives the piston before it and pushes the crank which is concealed in the lower end of the cylinder half-way around. The crank is attached to the shaft, which carries the fly-wheel upon one end and the propeller wheel upon the other end. The momentum of the moving parts—chiefly that of the fly-wheel—suffices to accomplish the remaining half of the revolution.
That any machine could be devised which could repeat these four events 700 times a minute was unthinkable a few years ago.
The first men who thought that a gasolene engine could be a practical thing were considered visionaries, but now they are found to be more practicable than steam engines. They are so efficient that they compete with the steam engine upon its own ground, and, in addition, they have opened up regions of usefulness which the steam engine can never exploit. So far as we can see, they have a permanent monopoly of the navigation of the air.
It is with the fourth event mentioned above, viz., kindling the explosive mixture, that we are now concerned. The high temperature required for this is obtained by forcing an electrical current against resistance.
Fig. 113
Five dry battery cells would very readily heat a short piece of fine wire to a sufficiently high temperature to explode the mixture, but it is impossible to alternately heat and cool a wire twelve times a second. It is too slow an operation. The only other method known at present is to imitate the lightning and force an electric current against the resistance of the air with sufficient power to produce the required heat. This, however, requires an extremely high voltage—at least 5000 volts, and our battery of five cells has not more than seven and a half volts of pressure. The interesting question then is, how does the spark coil enable us to raise the voltage from 7 to 5000.
To help toward an understanding of the matter I took seven small wire nails which I found in the boat—they were sixpenny finishing nails. I then took two or three yards of No. 24 insulated magnet wire, such as is used upon electric bells, etc. I use it more often than any other wire, and always have some about the boat. I fastened one end of this wire to one of the binding posts of a dry cell ([Fig. 113]), a, and attached branches c and d to it. The other end, b, was left free to act as a switch for closing the circuit by touching it to the remaining binding post.